Williams, a 15-time Grand Slam champion, seeks her first French Open title since 2002. She hasn't reached the semifinal 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.
"I play very aggressive," Williams said. "That's important for me, because I want to keep on winning here."
The Sharapova-Bouchard match was suspended on Thursday night with Sharapova up a break in the second set at 4-2.
Sharapova only had the tiniest bit of trouble closing things out Friday, erasing a couple of break points in the final game. On the second, 2012 Wimbledon junior champion Bouchard wasted a chance to get to 5-all when she flubbed a backhand long, then covered her face with her hand.
Sharapova took the next two points.
She completed a career Grand Slam by winning the French Open last year, adding to titles at Wimbledon in 2004, the U.S. Open in 2006 and Australian Open in 2008.
"It was such a long day yesterday, and obviously it's always difficult to have to come back," Sharapova said.
No. 12-seeded Maria Kirilenko defeated wild card Ashleigh Barty 6-3, 6-1. No. 13 Marion Bartoli of France delighted a partisan crowd on center court by beating qualifier Mariana Duque-Marino 7-6 (5), 7-5.
Information from The Associated Press was used in this report.