Rafael Nadal was up a set and had a break point against defending champion Milos Raonic when he sent a forehand just wide.
It was a mistake the 14-time Grand Slam champion wouldn't recover from.
Raonic made the most of the reprieve, holding serve in that fifth game of the second set and then attacking Nadal's serve in the eighth to swing the momentum his way in a 4-6, 6-3, 6-4 quarterfinal win Friday at the Brisbane International.
The top-seeded Raonic broke Nadal's serve again to start the third set, and calmly held on for only his second win in eight matches against the Spaniard.
Raonic served 23 aces and hit 50 winners to just 19 for Nadal, who could only convert one of his seven break-point opportunities.
As well as the big, deep service returns, Raonic also repeatedly went to the net, trusting his instincts and putting pressure on Nadal.
"Today the mentality behind the match was what sort of kept me around," Raonic said. "Some moments things weren't looking great. I wasn't efficient coming forward. I was missing some shots I shouldn't be. I was rushing.
"But at least I kept myself there, and I was able to always recuperate the next point. That's what I have to be most proud of."
Nadal, coming back from a layoff after an injured left wrist curtailed the end of his 2016 season, beat Raonic in an exhibition tournament last week. But Raonic played with more intensity in Brisbane, and Nadal said a couple of lapses were costly.
"Probably if I put that passing shot forehand cross, I had the break in the second set, big chance that we will be here one hour before with a victory," Nadal said. "That passing shot was long, and that's it. Then he had the break and match changes."
Nadal said three wins at the exhibition tournament, two wins and a close result in Brisbane gave him confidence his progress was good ahead of the Australian Open, where he is desperate to make amends for a surprising first-round exit last year.
Still in contention to start back-to-back seasons with a Brisbane title, Raonic will play seventh-seeded Grigor Dimitrov — a 6-3, 4-6, 6-3 winner over No. 4 Dominic Thiem — in the semifinals.
U.S. Open champion Stan Wawrinka and third-seeded Kei Nishikori will meet in the other semifinal match.
The second-seeded Wawrinka beat unseeded Kyle Edmund 6-7 (2), 6-4, 6-4 to reach the semifinals in his first trip to the Brisbane tournament.
In the previous three years, Wawrinka won the title in Chennai in the first week of the season before heading to Australia for the season's first major.
Wawrkina has a 4-3 lead over Nishikori in career head-to-heads, including the semifinals at the U.S. Open last year, but Nishikori won two of the three meetings in 2016.
Nishikori has now reached the semifinals four times in seven visits to the Brisbane International, needing just an hour for a 6-1, 6-1 quarterfinal win over Australian wild-card entry Jordan Thompson.
"I think I played one of the best matches so far, really dominating from the baseline and serving good today," Nishikori said. "Everything was working well."
U.S. Open finalist Karolina Pliskova will play Alize Cornet in the women's final on Saturday.
Cornet was leading 4-1 when French Open champion Garbine Muguruza retired with a right thigh injury. Pliskova beat sixth-seeded Elina Svitolina 6-2, 6-4 in the other semifinal match.
"A little bit of luck never killed anybody," said Cornet, who finished last year ranked No. 46 but now expects to be seeded at the Australian Open. "I'm just going to take it. I really enjoy the fact that I'm in the final. It's a big day for me, yeah."
Muguruza didn't think the injury setback would trouble her at the Australian Open.
"It will not stop me," Muguruza said. "Cornet was playing good. I couldn't match her level today. I had some pains, and I thought it was smarter to take care of my body."