Paris, Jun 2 (IANS/EFE): US tennis player Serena Williams, ranked No.1 in the world, reached the Roland Garros quarter-finals after beating her compatriot Sloane Stephens 1-6, 7-5, 6-3 in two hours.

Despite losing the first set against the 40th-ranked Stephens, Serena will press forward in the quarter-finals to chase after her 20th Grand Slam title.

She won the tournament in in 2002 and 2013.

Williams, 33, also had a tough time in the first set of her previous matches as well, conceding to Belarusian player Victoria Azarenka and also to Germany's Anna-Lena Friedsam in the second round.

"Not simple for me today. I'm surprised to win. Experience helped me," Williams said after the game, adding that her 22-year-old opponent played very well, and noted that she would have to start playing faster.

To reach the fourth Roland Garros semi-final of her career, Williams will have to eliminate Italy's Sara Errani, who defeated German Julia Goerges 6-2, 6-2 and in turn advanced to the French quarter-finals for the fourth year in a row.

Errani, who reached the Roland Garros final in 2012, has failed to defeat Serena in all seven of their previous encounters, one of which was in Paris on clay during the 2013 semi-finals.