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 semi-finals, Williams faced another familiar foe in Stosur, who had beaten her in straight sets in the 2011 US Open final.

Williams has won all four meetings against the Australian since but the 17-times grand slam winner was made to work much harder on the Cincinnati hard courts than she did in Montreal last week where she pounded Stosur 6-0 6-2.

In a tight contest that did not feature a single break, Williams needed nearly two hours to advance to the last 16.

“She was up in both of the (tie)breakers, I think it was just a great match, to be honest,” said Williams, who has never lifted the Cincinnati trophy.

“She served unbelievable, and I was like, I can’t lose serve because she’s just serving great. It was really intense.”

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

Elina Svitolina provided the biggest upset by stunning Wimbledon champ Petra Kvitova 6-2 7-6.

Third round: Serena Williams bt Pennetta 6-2 6-2.