World No 7 Dominic Thiem set up an Indian Wells Masters semi-final showdown with Milos Raonic without hitting a ball Thursday as injured Gael Monfils withdrew.
France's Monfils, ranked No 19 in the world, pulled out shortly before his scheduled quarter-final against Thiem with left Achilles tendon trouble.
Oft-injured Monfils said he felt discomfort in his ankle for a couple of days and had been receiving treatment, but the pain was worse after his fourth-round win over Philipp Kohlschreiber on Wednesday night.
"I tried a little jog this morning, and I couldn't really actually push on it," he said. "Then we taped it. Then I hit a little bit tonight, and I felt that I cannot be 100 per cent. I have this pain, and it wasn't the right choice to play."
He said an ultrasound exam confirmed severe inflammation, and he wasn't yet sure if it would prevent him from playing the upcoming Miami Masters.
"Very disappointing, to be honest," said Monfils, who was enjoying a 15-3 start to the season. "Because I was playing great tennis, feeling strong."
Thiem is through to the semi-finals at Indian Wells for the first time, having reached the quarters once before in 2017.
"I was really surprised," Thiem said, who was waiting to be taken to the court when he realized there was a problem.
"I'm really sorry for him. Obviously we are both playing well here. He's having a great season. I'm picking up my form again. So I think it would have been very tough but also a very fun match for us, for the crowd. So it's a pity," said Thiem who has lost both of his prior matches against Raonic.
Raonic ends Kecmanovic's dream run
Raonic ended the dream run of lucky loser Miomir Kecmanovic, beating the 19-year-old Serbian 6-3, 6-4.
Raonic, ranked 14th in the world, fired 13 aces and broke Kecmanovic once in each set to keep his bid to better his 2016 runner-up finish on track.
Kecmanovic, who failed to make it through qualifying but gained entry to the main draw when fifth-seeded Kevin Anderson withdrew, was the first lucky loser to reach the quarter-finals at Indian Wells since the tournament served as the inaugural ATP Masters 1000 in 1990.
Raonic's serve wasn't at it's booming best: he put only 55 per cent of his first serves in play.
"I think that can get better," Raonic said. "But there's other things I'm doing well, for sure. As soon as I had a chance on the first forehand, I was able to find my way to be the one dictating the point. And I put a lot of returns in and gave myself a chance there."
While the 130th-ranked Kecmanovic fell short in his bid to become just the third lucky loser to reach the semi-finals of a Masters 1000 event, he can regroup for the upcoming Miami Masters - where he has been given a wild card into the main draw.
Roger Federer and Rafael Nadal will be vying to set up a blockbuster semi-final when the play in the other two quarter-finals on Friday.
Federer, the 20-time Grand Slam champion who claimed his 100th career title at Dubai last month, takes on 67th ranked Hubert Hurkacz of Poland while Nadal faces 12th-seeded Russian Karen Khachanov.