Roberta Vinci of Italy serves to Serena Williams of the US during their women's singles match at the French Open tennis tournament at the Roland Garros stadium in Paris, June 2, 2013. [Photo/Agencies]

Vinci, who has won three Grand-Slam doubles titles with last year's French Open singles finalist Sara Errani, tried to move her opponent around the court to compensate for her lack of firepower but it was ultimately a futile effort.

Although she put up a better fight in the second set, she came up short at crucial moments and lost the last 10 points in a row to succumb in one hour 10 minutes.

A sterner test for Williams is likely to come in the next round as her quarterfinal opponent is 2009 champion Svetlana Kuznetsova, who is not seeded at Roland Garros for the first time since 2003 but beat the American when they last played at Roland Garros.

The Russian dropped to 85th in the world after a six-month injury layoff following last year's Wimbledon, but has climbed to 39 and is a walloping baseliner who can crunch winners off both flanks.

"The last time we played here she won, so that will probably get her pumped up and she's on a comeback," Williams said. "She has nothing to lose but everything to gain."

Kuznetsova booked her place in the quarterfinals by outplaying German eighth seed Angelique Kerber 6-4 4-6 6-3.

"She's the best in the world so far. She's been playing unbelievable tennis," Kuznetsova said of Williams. "But I believe that I have my game and my good days, as well. Let's cross fingers I will have a good day that one. She's just a fighter."