Rafael Nadal has reached the final of the French Open after opponent Alexander Zverev was forced to retire injured.

In distressing scenes at Roland Garros on Saturday, German third-seed Zverev fell and twisted his ankle at the end of the second set, yelling in pain as Nadal rushed around the net to help.

Zverev was helped to his feet but taken off the court in a wheelchair for treatment.

After five minutes, the 25-year-old re-emerged on crutches to inform the umpire that he could not continue.

The crowd on Philippe-Chatrier — fiercely pro-Nadal — rose to salute the world number three as his latest bid for a grand slam title came to a devastating end.

"It's very tough, very sad for him," said Nadal.

"He was playing an unbelievable tournament, and I know he was fighting to win a grand slam.

"I'm sure he will win more than one and I wish him all the best."

On his 36th birthday Nadal, broken in the very first game, was behind the eight ball for most of the opening set.

He was standing as far back in the court as possible as he defended Zverev's thunderous serve. 

Having clawed the break back, he found himself facing four set points at 6-2 down in the tie-break but somehow hit back, including one improbable backhand followed by an outrageous forehand to snatch the game 10-8 after 92 gruelling minutes.

Nadal, after taking the first set 7-6 (8), was up against it once more in the second. The last thing either player needed was a 44-shot rally, but that is what they got early in the second set among four breaks of serve in the opening games.

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The double-faults which cost Zverev at crucial times in the first set returned to gift Nadal two more breaks, the second coming when he was serving for the set, moments after he had been given a warning by the umpire for swearing in Russian.

When the Spaniard levelled again at 5-5, it was the first time he had held serve for 93 minutes.

But at 6-5 in the second, and with Nadal about to take the match into a second tie-beak, Zverev's foot gave way.

The world number three yelped in pain and Nadal rushed around the net to check on his opponent as he was attended to by medical staff.

Nadal had made it through - not in the way he would have liked, of course - but he remains on track for Roland Garros title number 14 and a 22nd grand slam win.

He will take on Casper Ruud, who became the first Norwegian to reach a grand slam final after defeating Marin Cilic 3-6 6-4 6-2 6-2 in Saturday's other semifinal.