Rafael Nadal reached his 14th French Open final on Friday when Alexander Zverev was forced to quit their semi-final after suffering a sickening right ankle injury.

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 horror injury.

As his piercing screams of pain echoed around the 15,000-seater court, a tearful Zverev was helped from the court by medics.

Minutes later, the 25-year-old returned on crutches and conceded the match, with Nadal embracing his heartbroken opponent.

All over: Alexander Zverev walks with crutches back on to the court
All over: Alexander Zverev walks with crutches back on to the court AFP / Anne-Christine POUJOULAT

"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 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."

He added: "For me, to be in the final of Roland Garros is a dream, without a doubt, but at the same time, for it to finish that way... I have been there in the small room with Sascha and to see him crying like that -- I wish him all the best."

Nadal, chasing a record-extending 22nd Grand Slam title, will play either Marin Cilic or Casper Ruud in Sunday's final.

Moment of horror: Alexander Zverev on court as a concerned Rafael Nadal looks on
Moment of horror: Alexander Zverev on court as a concerned Rafael Nadal looks on AFP / Anne-Christine POUJOULAT

Until the dramatic end of the match, Zverev had been pushing Nadal all the way.

In a rollercoaster, 91-minute first set, Zverev broke in the opening game before Nadal levelled at 4-4.

The Spaniard, dripping with sweat under the closed roof on Court Philippe Chatrier, saw three set points come and go in the 10th game as the German's all-or-nothing hitting kept him in the match.

Nadal, celebrating his 36th birthday Friday, then saved four set points in a knife-edge tiebreak, one of them off a spectacular running crosscourt forehand.

A blistering forehand pass gave him the opener on a sixth set point.

Zverev, playing in the semi-finals at Roland Garros for the second successive year, hit 25 winners and 26 unforced errors in the set.

The second set was punctuated by eight breaks of serve. When Nadal broke for 2-1, he did so on the back of an exhausting 44-shot rally.

Zverev also collected a warning for bad language as the set headed towards another tiebreak.

However, that became quickly irrelevant when his attempt to reach a second Grand Slam final ended in such dramatic circumstances.

Nadal becomes the second oldest man to make the final in Paris after 37-year-old Bill Tilden who was runner-up in 1930.

Victory gives Nadal, who knocked out world number one and defending champion Novak Djokovic in the quarter-finals, a 111-3 record at the French Open.