Defending champion Maria Sharapova has been knocked out of the French Open by 13th-seed Lucie Safarova.

The second-seed was beaten 7-6, 6-4 in the fourth round on an overcast day.

It is Sharapova's earliest defeat at Roland Garros since 2010, when she was beaten in the third round.

Since then, the Russian won the clay-court Grand Slam tournament in 2012 and 2014, was the runner-up to Serena Williams in 2013, and reached the semi-finals in 2011.

In her first French Open quarter-final, Safarova will face 21st-seed Garbine Muguruza of Spain, who beat Flavia Pennetta of Italy 6-3, 6-4.

Top-seed Serena Williams continued to struggle through the French Open draw, huffing and puffing her way into the quarter-finals with a 1-6 7-5 6-3 win over fellow American Sloane Stephens.

Stephens broke serve three times in the opening set against the 2002 and 2013 champion.

Williams, the 19-times grand slam winner, broke serve twice in the second set to level.

Stephens then faded away as Williams broke for a 3-2 lead in the decider before finishing the match on Stephens' serve with a dipping crosscourt backhand winner.

She next faces 17th seed Sara Errani of Italy for a place in the semi-finals.

Results:

Alison Van Uytvanck (Belgium) beat Andreea Mitu (Romania) 6-1 6-3

1-Serena Williams (U.S.) beat Sloane Stephens (U.S.) 1-6 7-5 6-3

17-Sara Errani (Italy) beat Julia Goerges (Germany) 6-2 6-2

13-Lucie Safarova (Czech Republic) beat 2-Maria Sharapova (Russia) 7-6(3) 6-4

21-Garbine Muguruza Blanco (Spain) beat 28-Flavia Pennetta (Italy) 6-3 6-4

Safarova will be taking part in her first French Open quarter-final [Reuters]

Source: Al Jazeera and agencies