By John Stonestreet

PARIS (Reuters) - World number ones Novak Djokovic and Serena Williams will feel like they are working overtime on Friday when organizers cram all four semi-finals into a single session to try and ensure a rain-ravaged tournament ends on schedule.

The Serbian will be on court for a fourth day running and the American for a third, after the players' usual rest days between matches were eaten up by persistent rain delays that washed out Monday and meant just two hours of play were possible on Tuesday.

The Serbian will also have to trade down to Roland Garros' number two court to battle past his young Austrian opponent, Dominic Thiem. The 13th seed has lost both their encounters but is strengthening his case with every win to be viewed as the pick of a new tennis generation.

"He's playing the best tennis of his life," Djokovic told a news conference on Thursday after dispatching Czech Tomas Berdych in straight sets.

"He's played a lot of matches, especially on clay, which is his favorite surface... I'm sure he's going to give it all in the semis."

Weather permitting, all semi-finals at grand slam tournaments are invariably played on the main showcourt, but the scheduling backlog means Djokovic and Thiem, and before them Spanish women's fourth seed Garbine Muguruza and Australian Sam Stosur, will play Friday's matches on Court Suzanne Lenglen.

Djokovic, who will be contesting his 30th grand slam semi-final, is not a complete stranger to playing a last-four showdown away from the main stage. In 2007, his Wimbledon semi-final against Rafael Nadal took place on Court One.

Taking pride of place on Paris' main Philippe Chatrier showcourt will be Williams and unseeded Dutchwoman Kiki Bertens, followed by arguably the most mouthwatering clash of the day between world number two Andy Murray and defending champion Stan Wawrinka.

"I think he's been an amazing player for many, many years," Wawrinka told reporters about Murray, who has won eight of their 15 encounters.

"He always tries to be a better player. The way he's improved on clay is quite impressive, because now he's winning a lot of tournaments."

Both the Swiss and the Briton have had the luxury of an extra day's rest, having completed their fourth-round matches on Sunday before the rain set in.

But while Djokovic, who along with Thiem did not clear the fourth round until Wednesday, is keenly aware he has had less time to recharge his batteries, that may just make him concentrate all the harder.

"The way that the schedule has been going on in the second week means there is not much time to really reflect on what you have done," he said.

"My focus right away goes for recovery and the next match."

(Editing by Pritha Sarkar)