Williams had 10 aces and three service winners late in the tiebreak. The 12-time Grand Slam champion also saved three break points late in the first set Monday.
Williams said she was displeased with her performance but declined to discuss specifics.
"I definitely didn't feel good about it," she said. "At least I won. I think I'm still in the tournament; that's what matters."
Williams won the title in Paris in 2002 after defeating her sister Venus in the final.
In other action, Caroline Wozniacki beat both the heat and an overmatched opponent. With temperatures headed into the mid-80s on a second successive cloudless day in Paris, the No. 3-seeded Wozniacki had a morning match on center court and needed barely an hour to dispatch Alla Kudryavtseva 6-0, 6-3.
"Doesn't matter if I play five hours or one hour," Wozniacki said. "As long as I win, I'm happy."
Wozniacki has been bothered by a sore right ankle since mid-April, and the injury forced her to retire from a match in Warsaw last Thursday, but she said it has much improved since.
"It feels pretty good today, actually," she said. "I've had good support in the tape, so I was really happy about that."
Wozniacki has played the most tennis of anyone on the women's tour since the start of last year, and she looked sharp, committing only nine unforced errors and missing just 11 first serves. Kudryavtseva lost the first seven games before winning two in a row, but she was unable to stay with Wozniacki in long rallies.
Kudryavtseva changed rackets before the final point, but it was too late. Wozniacki closed out the victory with a service winner and then blew kisses to the crowd.
She said she's glad to be one of the highest-seeded players.
"You get to play on the big courts, nice crowd. It's just a great thing," Wozniacki said. "It means that you have done so well throughout the year and you've done so many good results. I'm just happy when I go on court."
The women's draw is considered wide open, with Wozniacki among the title contenders. The 19-year-old Dane was the runner-up at last year's U.S. Open to Kim Clijsters and has a 29-12 career record in Grand Slam tournaments.
Other winners Monday were 2008 winner Ana Ivanovic, No. 4 Jelena Jankovic, fifth seed Elena Dementieva, seventh seed Samantha Stosur, No. 8 Agnieszka Radwanska, No. 11 Li Na, 17th seed Francesca Schiavone, No. 27 Alona Bondarenko, 31st seed Alexandra Dulgheru and No. 32 Kateryna Bondarenko. Anabel Medina Garrigues of Spain also advanced with a 7-5, 6-2 victory over American Melanie Oudin.
Oudin lost in qualifying at Roland Garros a year ago, but her ranking of 37th now allows her direct entry into tournaments -- meaning she keeps gaining experience at tennis' top levels.
"I'm definitely used to it a lot more because I've been in this environment more now than I was before," Oudin said. "I still love playing, and I think that's the important thing -- that I'm enjoying it, looking forward to going to new places that I get to go this year."
Information from The Associated Press was used in this report.