PARIS, June 4 -- American tennis great Serena Williams pulled out of the 2018 French Open due to injury on Monday, a moment ahead of her women's singles fourth round match against Russia's Maria Sharapova.

Williams, who made a return to Grand Slam tournament after giving birth to a daughter last September, announced her withdrawal at a press conference.

"Unfortunately I've been having some issues with my pec muscle. Right now I cannot actually serve, so it's hard to play when I can't physically serve," said Williams, adding that she will wait for the results of Tuesday's MRI and consultation to doctors before making any decisions about future training plans and Wimbledon.

Williams had won 18 straight matches against Sharapova prior to their 22nd head-to-head clash on Monday.

Sharapova will take on third-seeded Garbine Muguruza, who advanced to last eight following Lesia Tsurenko's retirement in the first set due to injury.

Top seeds Rafael Nadal and Simona Halep won in straight sets to secure their places in the quarterfinals.

Despite conceding a break at the start, 10-time champion Nadal swept Roland Garros debutant Maximilian Marterer of Germany 6-3, 6-2, 7-6 (4).

The Spaniard will next face Argentine Diego Schwartzman, who rallied from two sets down to eliminate No. 6 seed Kevin Anderson in full sets.

"It's a match against one of the best players of the world today, so what you expect is a very tough match and you need to be ready to accept that and fight for it," said Nadal.

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