Rafael Nadal reached his 14th French Open final when Alexander Zverev was forced to quit their semi-final after suffering a serious right ankle injury on Friday.
Zverev had to leave the court in a wheelchair after turning his ankle as he chased down the ball in front of the players' boxes on Court Philippe Chatrier where his team and family were sitting.
The German was trailing 7-6 (10/8), 6-6 after more than three hours of play at the time of the injury. A tearful Zverev was then helped from the court by medics.
Minutes later, the 25-year-old returned on crutches and conceded the match, with Nadal embracing his opponent.
"It's very tough and very sad for him. He was playing an unbelievable tournament and he's a very good colleague on the tour," said 13-time champion Nadal.
"I know how much he's fighting to win a Grand Slam. For the moment, he was very unlucky. I'm sure he'll win not one, but much more than one. I wish him all the best.
"It had been a super tough match. Over three hours and we didn't even finish the second set. It's one of the biggest challenges on the tour when he's playing like he did today."
Nadal will take on Casper Ruud in the title match after the eighth seed rallied from a set down to beat an error-prone Marin Cilic 3-6, 6-4, 6-2, 6-2 on Friday to become the first Norwegian man to reach a Grand Slam singles final.
With his victory against the Croat, Ruud will rise to a career-high world ranking of sixth.
"It was a great match from my side. I did not start great but from that break [second set] I played some of my best tennis this year," Ruud said in his on-court interview.
"Marin is usually the one who is playing very fast and playing the balls very hard. I figured I need to step up a bit and counter attack and go for some faster shots. That helped."
Ruud fired 16 aces and 41 winners past 2014 US Open champion Cilic.
However, the match will be remembered for a serious security breach as environmental activist interrupted the semi-final for 15 minutes.
The woman, a French citizen, jumped onto Court Philippe Chatrier and tied and glued herself to the net. She wore a t-shirt that read 'we have 1028 days left' tied herself to the edge of the net with a string around her neck.
The message, written on both sides of her white t-shirt, was a reference to a UN report on climate change.