Williams maintained her perfect record against Schnyder

Williams, playing just her third tournament of the year after struggling with a wrist injury, won 3-6 6-3 6-4.

The 26-year-old was a break down in the final set before fighting back to take her record against Schnyder to 9-0.

The unseeded Williams, currently ranked world number 32, will now play fellow American Meilen Tu in the third round.

Second seed Jelena Jankovic thrashed Czech qualifier Iveta Benesova 6-1 6-2 in her opening match.

The 22-year-old Serbian moves into the last 16 and a match against Alona Bondarenko of Ukraine.

It was one of those days where I felt I was just a little bit off Venus Williams

Croatia's Karolina Sprem had a fine 6-3 6-3 win over seventh seed Li Na of China, while eighth seed Tatiana Golovin beat Catalina Castano 6-0 6-3.

Other seeds who made it through to the next round included Tatiana Golovin, Samantha Stosur, Sybille Bammer and Alona Bondarenko.

Williams admitted that her previous good record against Swiss player Schnyder had helped her.

"Going to a match for her against an opponent you've never won against, I think that's tough," she said.

"That probably played a role. She was probably nervous, too. She was hitting some double faults at times when there was no need to.

"It was one of those days where I felt I was just a little bit off, and against a player like Patty, you need that extra bit.

"I felt I was sometimes hitting in the open court too much and that some of the balls were floaters. That's why I went more to a serve and volley attack in the third set."