How Del Potro can be a 'nightmare' for Nadal...

'He is tall so Rafa's shots don't bounce over his shoulder, he has big weapons'

Juan Martin del Potro could be a 'nightmare opponent' for Rafael Nadal when they clash in the French Open semi-finals on Friday, according to former champion Mats Wilander.

The 29-year-old Argentine, close to his best after battling back from multiple wrist surgeries, reached his first French Open semi-final since 2009 when he downed Croatia's Marin Cilic in four sets on Thursday.

Nadal has been untroubled en route to the semis, apart from a blip in the quarter-finals against Diego Schwartzman, but the towering Del Potro looms large on Court Philippe Chatrier on Friday.

Del Potro has served 53 aces to reach the semi-final and his forehand, a stroke that could chop down trees, worries the very best as he showed by beating Roger Federer at last year's US Open and winning the Indian Wells title in March.

He has also had success against 10-time French Open champion Nadal, winning five of their 14 matches although never on the Spaniard's favoured red dirt.

They have not played on clay for 11 years, however, so it will be a step into the unknown on Friday.

"He is a nightmare opponent for Nadal," three-time French champion Wilander, hosting Eurosport's flagship Game, Schett and Mats programme at Roland Garros, told Reuters.

"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 problem for Del Potro might be his backhand which has lost a little sting since his wrist surgeries.

"Tomorrow I will try to play a match as I did in a different surface against Rafa," Del Potro told reporters.

"But he's lefty and he can find easily my backhand, and he knows what my backhand is at the moment. I will try to do my best, to play my best tennis, and see if I can do a good match."

Friday's first semi-final has 72nd-ranked Italian Marco Cecchinato up against Austrian seventh seed Dominic Thiem, the only player to beat Nadal on clay this year and who is bidding to reach his first Grand Slam final after consecutive semis in Paris.

While Thiem is a heavy favourite, Cecchinato, 25, will be completely unafraid, Wilander believes.

"He's never been there before, it's a big surprise but he didn't fall into the semi-finals, he beat David Goffin and Novak Djokovic and he has played really well," he said.

"Could he beat Nadal in the final, probably not, but Nadal is not guaranteed to get to the final."

Thiem has played with great authority and Wilander thinks he could also worry Nadal in the final, should they meet.

But first he must step into the unknown.

"It's easy to get to where you've been before but harder to go one step further," Wilander said.

"Can he play with pressure? I'm not sure. He throws in matches sometimes so he might not play his best. But he is becoming tactically more correct and five sets will suit him."

"If he doesn't win this match people, if he can't beat Cecchinato people will say he's never going to win a Slam. The pressure is 95 per cent on him."