World number one Serena Williams won a tense battle of US Open champions by beating Australia's Samantha Stosur 7-6, 7-6 to reach the third round of the Western and Southern Open in Cincinnati.

After last week's loss to sister Venus in the Montreal seni-finals, Williams faced another familiar foe in Stosur, who had beaten her in straight sets in the 2011 US Open final.

Elsewhere, fourth-seeded Pole Agnieszka Radwanska, fresh from her Rogers Cup win in Montreal on Sunday, cruised to a 6-2, 6-2 win over Japan's Kurumi Nara, while German sixth-seed Angelique Kerber beat Russia's Ekaterina Makarova 6-4, 6-1.

Ukrainian teenager Elina Svitolina provided the biggest upset of the day by stunning Wimbledon champion Petra Kvitova 6-2, 7-6.

"I have to say number one I'm disappointed how I played today, for sure," said Kvitova. "From the beginning I didn't feel very comfortable out there and I was trying to really fight. I just wasn't able to do that today. I'm not pretty sure why."

Canadian Eugenie Bouchard, seeded-seventh, was ousted by Svetlana Kuznetsova, the Russian winning 6-4, 3-6, 6-2 to make it two from two in her career against Bouchard.

In the other evening matches, Ana Ivanovic easily took out American wild card Christina McHale 6-4, 6-0 while Jelena Jankovic was too good for German qualifier Annika Beck 6-1, 7-6.

Source: Reuters