Spaniard Nadal, chasing an 11th title at Roland Garros, recovered from a sluggish start against a confident Maximilian Marterer as he ousted the German 6-3, 6-2, 7-6.

The 32-year-old 16-time Grand Slam winner will next face Argentinian 11th seed Diego Schwartzman, who fought from two sets down to beat Kevin Anderson of South Africa 1-6, 2-6, 7-5, 7-6, 6-2.

The story of the day came off the court, however, as Williams confirmed she would not be able to play her match with Sharapova due to injury.

Williams, who has 23 Grand Slam titles to her name, recently returned to tennis after giving birth to her first child.

She looked in good form despite her return but the American said she was unable to continue at the tournament owing to an arm problem which was affecting her serve.

Her decision denied the crowd, and the tennis world, the chance to see her take on Sharapova in what was billed as a titanic encounter.

Williams admitted she now faces a race against time in order to be fit for the grass court season and Wimbledon.

"I'm beyond disappointed," she said. 

"I gave up so much from time with my daughter and time with my family all for this moment. 

"So it’s really difficult to be in this situation."

Sharapova progresses straight through to the quarter-finals as a result of Williams' withdrawal.

The Russian, a two-time French Open champion, will face either 2016 winner Garbine Muguruza or Ukraine's Lesia Tsurenko in the last eight.

Elsewhere in the women's draw, world number two Caroline Wozniacki of Denmark was beaten in straight sets by Daria Kasatkina of Russia.

Kasatkina produced a superb display as she knocked out Wozniacki by virtue of a 7-6, 6-3 triumph.

Top seed Simona Halep of Romania is also through after she dispatched Belgium's Elise Mertens 6-2, 6-1.