Rafael Nadal progressed to his 11th French Open semi-final with a four set victory over Diego Schwartzman on Thursday and the Spaniard has thus far never lost once he makes the last four at Roland Garros.

Nadal dropped a set against the Argentine, the first this tournament, but came back fighting to win 4-6, 6-3, 6-2, 6-2 in a match that was interrupted by rain and had to be completed a day later. He later admitted the rain certainly came to his aid as he was not playing at his best when the match got underway Wednesday.

The world number one is still the overwhelming favorite to pick up his 11th French Open title, but former winner Mats Wilander thinks Juan Martin del Potro, Nadal’s semi-final opponent could cause him some troubles.

It will be Nadal’s second Argentine opponent in as many rounds after Del Potro defeated Marin Cilic 7-6 (7-5), 5-7, 6-3, 7-5 in the other quarter-final to book his place in the last four. It is his best result at Roland Garros since 2009 when he also made the semi-finals losing to Roger Federer in five sets.

Del Potro has a torrid time with injuries in recent years and the last 12 months have seen the Argentine trying to recapture the form that took him to his first and only Grand Slam title at the US Open in 2009. He looks like he is almost back to his best this season having picked up two titles, which includes winning his first Masters Series title with a three set win over Federer at Indian Wells.

Rafael Nadal
Rafael Nadal progressed to his 11th French Open semi-final on Thursday. In this picture, Spain's Nadal celebrates after victory over Argentina's Diego Schwartzman during their men's singles quarter-final match on day twelve of The Roland Garros 2018 French Open tennis tournament in Paris, June 7, 2018. THOMAS SAMSON/AFP/Getty Images

Despite preferring hard courts, he has won four clay court titles during his career and possesses a powerful game that Wilander thinks can trouble Nadal. Moreover, the Swedish former world number one thinks the Argentine can use his height to his advantage to negate the Spaniard’s looping shots.

“He is a nightmare opponent for Nadal,” Wilander said about Del Potro, as quoted on Tennis World USA. “He portrays someone who believes in himself as much as Federer, more than (Novak) Djokovic and more than (Andy) Murray.”

“Sometimes he gets fired up but has this calm about him that I think worries Nadal and Federer. They think, what’s he up to? Has he figured me out? … He is tall so Rafa’s shots don’t bounce over his shoulder, he has big weapons, the serve which gives him free points and the forehand which is the biggest in the game and he can hit through Nadal’s forehand with it. Delpo is not afraid of too many players in the world,” the seven-time men’s singles Grand Slam winner added.

The men’s singles semi-final at Roland Garros will take place Friday with Nadal taking on Del Potro in the first semi-final, while the other semi-final will see second favorite Dominic Thiem take on surprise candidate Marco Cecchinato, who beat Novak Djokovic in the quarterfinal.