A star is born

In a few hours, Denis Shapovalov will head to the airport for his flight to New York for the U.S Open. But at the moment, he's strolling into the reception area of Tennis Canada's Montreal headquarters wearing a giant multicoloured sombrero.

The 18-year-old rising Canadian tennis star greets me warmly, shakes my hand and asks about my travels. The rain in Montreal has washed out his last practice at Uniprix Stadium, so he says he's spent his final morning of preparations at the National Training Centre working on his fitness. There's a story behind this festive Mexican hat, but he's keeping me in suspense.

He directs me toward a lounge area, where we settle in for a lengthy interview. The precocious youngster is a week removed from his life-changing run to the semi-finals at Montreal's Rogers Cup, a wild spree that included upsets over Grand Slam champs Juan Martin del Potro and Rafael Nadal. The ride catapulted him up 76 spots in the ATP world rankings, captivated a national audience and turned heads on the Tour. But Mr. Shapovalov is already focused on the next big test.

On Tuesday, he begins his quest to win the three qualifying matches needed to earn his way into the main draw of the U.S. Open. Many will be watching him with intrigue. Was his Rogers Cup ride just a fluke? Can this kid hold the No. 69 ranking on the ATP Tour? Is he ready to compete regularly with the world's best tennis players earlier than anyone projected?

Playing five ATP matches in five days at the Rogers Cup was unlike anything his young body had been through before. He won't soon forget the intense muscle cramping he experienced while soaking in an ice bath after that lengthy duel with Mr. Nadal or his team scrambling to put salt in his drinks to help alleviate the pain.

He chose to pull out of an ATP Challenger tournament for which he was scheduled the next week in Vancouver. Instead, he rested for a couple of days in Richmond Hill, Ont. with his family, then went right back to Montreal to train with coach Martin Laurendeau. If he gets through qualifying in New York, he must be ready for the rigours of best-of-five Grand Slam matches.

"Those five Rogers Cup matches were physically tough, and there were late nights, up until 2 or 3 a.m – how could I fall to sleep after beating some of my heroes?" Mr. Shapovalov says. "There was cramping, my legs were so heavy and my body was exhausted. Every day was a pressure situation. Once the final point was played, I really felt everything that my body had been through. Yeah, I'm fit, but I need to get even more fit to compete with these guys every week."

Mr. Shapovalov takes off the sombrero and gives his blond shaggy hair a rustle, setting the straw hat aside knowingly, but still holding off on its story. In his purple Nike T-shirt, blue shorts and court shoes, he looks like many of the other kids milling around the lounge. Except he's the guy many are stopping to punch on the shoulder and say "Congrats, buddy" or "Man, that Rafa match!"

The gregarious teen repeatedly uses the word "ridiculous" to express the stunning examples of good fortune and attention he received while becoming the youngest ATP World Tour Masters 1000 semi-finalist since 1990. He's fired up that his Instagram followers spiked from 25,000 to 67,000 followers and his Twitter feed went from 6,000 to 23,000. His semi-final match with Alexander Zverev became the most-watched tennis match ever aired on Sportsnet, reaching 1.46 million Canadians. Roger Federer gushed about his game. Wayne Gretzky showed up to watch him play, and so did Olympic swimmer Penny Oleksiak, both lending to his sky-high motivation.

He shakes his head in disbelief as he gushes about Mr. Gretzky, a long-time friend of Mr. Shapovalov's manager, Andrzej Kepinski, and someone the young tennis player calls his "ultimate hero." Gretzky's youngest daughter, 14-year-old Emma, plays tennis, so Mr. Kepinski invited Mr. Gretzky and wife Janet to bring her to the Rogers Cup. Mr. Gretzky was impressed with Mr. Shapovalov, whom he'd met just once before at a Toronto Maple Leafs game.

"I learned a lot from observing Wayne at the Rogers Cup. He was offered a cart across the grounds, but he wouldn't take it; he walked and stopped and signed autographs for people, and I thought that was amazing," Mr. Shapovalov says. "He called me after I lost to Zverev and we talked. He told me how proud he was of me and how happy his family was to spend some time with me."

Ms. Oleksiak sat beside Mr. Shapovalov's mother Tessa in his player box during the Nadal match. It had many wondering about the connection between the tennis player and the 17-year-old Canadian swimmer who won four Olympic medals last summer at the Rio Olympics.

The two teens met in Toronto in February at the Conn Smythe Sports Celebrities Dinner for Easter Seals. They've kept in touch over text and Snapchat ever since, but their busy travel schedules have kept them from connecting much in person. Ms. Oleksiak was in the midst of a busy block of training sessions in Toronto but found one day off in her practice schedule to come to Rogers Cup.

"I was texting her all week to invite her and finally said, 'Listen, you're coming,' and so I called my manager to set it all up and we flew her in the morning of," Mr. Shapovalov says. "It was really nice to see her in my box during the match; she was part of my biggest win. It's always nice to have a close friend there for you in the big moments. We have a lot in common. She has the same problem as me with getting homesick while travelling to compete. It's nice to have someone to talk to about stuff like that."

Ms. Oleksiak, who agrees to a phone interview about Mr. Shapovalov the next day, echoes his sentiments.

"We're both athletes trying to progress in our sport and compete at the senior level despite still being really young, so we really understand one another, and we always have a lot to talk about," the swimmer says. "I can relate to what he's going through with all the media attention that comes your way all at once, and your name and face being all over TV. It's Denis everywhere right now. He's a super-good person and very grounded. We have the same kind of values. I want him to be one of my friends for a long time and I'm super excited for his success right now."

With the huge rankings jump, Mr. Shapovalov was the Tour's biggest mover of the week. He leap-frogged over fellow Canadian Vasek Pospisil (No. 78) to become the country's second-ranked male singles player behind No.11 Milos Raonic.

During a Rogers Cup week when Mr. Raonic lost his first match – as did Canada's top female singles player Eugenie Bouchard – Mr. Shapovalov's victories provided a nightly reason to pack Uniprix Stadium or gather around the TV. The lefty with the fiery on-court personality and sweeping one-handed backhand had provided intrigue before. He won the Junior Wimbledon last summer and following that up with an upset of then-No.19-ranked Nick Kyrgios at last year's Rogers Cup.

Since then, he won an ITF Futures tournament in Gatineau and Challengers in Drummondville and Gatineau. He also upset World No. 47 Kyle Edmund in an ATP 500 event in England.

When Rogers Cup granted him a wild card, he could have chosen to stay in a nice Montreal hotel during the tournament. Instead, he did what he always does when he visits Montreal to train, stay at the family home of another of Canada's highly touted teens – and one of his best friends – Félix Auger-Aliassime.

Mr. Shapovalov says 17-year-old Mr. Auger-Aliassime is "like a brother," and projects he, too, might have made headlines as a wild-card entry in this Rogers Cup had a wrist injury not caused him to withdraw. Much of the reason Mr. Shapovalov returned to Montreal in his lead-up to the U.S. Open was to train with Mr. Auger-Aliassime, who has since healed and will also try to qualify for New York, where he was the junior champion last summer.

Finally, he gets around to the sombrero.

Earlier in their training week, as crews were still cleaning up the grounds after the Rogers Cup, the two boys came upon the sombrero lying near the grandstand, presumably left behind by a fan.

"This sombrero was just sitting on the court – like it was waiting for us – so we decided to make a bet," Mr. Shapovalov says. "I told Felix, 'Let's play a set and the loser has to wear this to the airport to New York.' Felix didn't like that, so we said, 'Fine, the loser will wear it around here until we leave for the airport.'"

Mr. Shapovalov launches into an excitable point-by-point description of their set – making it sound as theatrical as one played between John McEnroe and Bjorn Borg in a Wimbledon final.

"Yeah, I watched them play for the sombrero, and believe me it was intense," Mr. Laurendeau would later corroborate. "Usually they play for 'butts up,' which means the loser turns around and bends down and the other gets to fire a serve across at his butt and that stings. But this was different. It was a hell of a tiebreak, very intense. Everyone had set points. That's the kind of dynamic they have. These two really push one another to get better."

Mr. Auger-Aliassime emerged the winner after a 10-8 tiebreak. Mr. Shapovalov was left to wear the big goofy party hat of shame.

It's not, however, doing much to dampen his mood. He looks, at the moment, like a kid on top of the world. That Rogers Cup run earned him $220,760 (U.S.) in prize money. There's been steady corporate sponsor interest in him since his Junior Wimbledon title, and he already has a handful of partnerships, including Nike, National Bank, BioSteel and Yonex. Kepinski says he's received lots more interest in the wunderkind recently but his team hasn't strayed from their plan to take it slow and not overburden him with obligations.

"We are not running after shiny objects," Mr. Kepinski says in a phone interview. "This kid does not want to be a flash in the pan. We know it's not going to get any easier for him now. Players have gotten a good look at his game now, they see how hard he hits and he's not going to be able to surprise people any more. No one on the ATP Tour wants to lose to an 18-year-old kid."

Mr. Shapovalov is in good position to make the Next Generation ATP Finals in Milan this November, an inaugural tournament for the Tour's top seven players under 21. He's now fifth in the Race to Milan rankings.

He expects his schedule will continue to include a mix of Challenger and ATP events for the next year or two. After the U.S. Open, Mr. Shapovalov hopes to be chosen for Canada's Davis Cup playoff tie versus India in Edmonton in September. He won't truly reap the rewards of his new ranking until early 2018, when he automatically earns a main draw bid to the Australian Open.

A young player and her father walk by and interrupt us, asking Mr. Shapovalov to pose for a picture. That's been happening more and more lately – in malls, on the street, everywhere. Our lengthy conversation comes to a close, and we say our goodbyes.

"Oh, one more thing," Mr. Shapovalov says, as he slides the sombrero back onto his head. "Don't you want a photo of this hat?"