Charleston: In a clash of former world number ones, Venus Williams toppled seventh-seeded Jelena Jankovic 7-5, 6-0 on Wednesday to reach the third round of the WTA's green clay Charleston tournament.

Jankovic had won her last four meetings on clay against Williams, but once the American heated up in the wake of a first-set rain delay, the Serbian couldn't stop her.

Jankovic served for the first set at 5-4, but Williams broke and never looked back.

By the time she won the second set she had reeled off nine games in a row, capping the match with a backhand winner down the line.

"I hadn't played her in a long time, so in the first set I was just finding my rhythm and getting used to any new patterns she might have," said Williams, playing on a wild card invitation as she tries to rebuild a ranking dented by a lengthy injury and illness layoff.

"In the second set everything started landing for me. I was moving forward and playing my aggressive game. Honestly, I don't know how it went that nicely for me. Everything just landed, and she hit a few errors, and that helped me a lot."

Russian Nadia Petrova, the 13th seed, saved two match points en route to a 2-6, 7-6 (8/6), 6-2 victory over US veteran Jill Craybas, a qualifier.

"Compared to my previous match in the first round I didn't play as well, but I have to give credit to Jill, she played very well today," said Petrova, who next faces eighth-seeded Anastasia Pavlyuchenkova. "She wasn't making any errors from her side, so I had to take control of the match and move her around, and I think I made too many errors."

Sixth-seeded German Sabine Lisicki, who claimed her first WTA Tour title at Charleston in 2009, rallied to beat Czech Andrea Hlavackova 2-6, 6-3, 6-4.

Lisicki next faces Yaroslava Shvedova, who upset 12th-seeded Belgian Yanina Wickmayer 6-2, 7-6 (7/2).

In other matches, Galina Voskoboeva of Kazakhstan defeated Argentine qualifier Paula Ormaechea 6-2, 6-3 to set up a third-round clash with Samantha Stosur.

New Zealand's Marina Erakovic, seeded 17th, beat Georgia's Anna Tatishvili 7-5, 6-3 to advance to a meeting with fifth-seeded American Serena Williams.