Williams, a 15-time Grand Slam champion, seeks her first French Open title since 2002. She hasn't reached the semifinals since 2003.
Ranked No. 1, Williams has won a career-best 27 consecutive matches. Since losing in the first round a year ago at Roland Garros, Williams is 70-3, including titles at Wimbledon, the U.S. Open, the London Olympics and the season-ending WTA Championships.
Against the No. 26-seeded Cirstea, Williams committed only 16 unforced errors. She has lost six games in three matches.