World number on Serena Williams won a tense battle of US Open champions by taming Australia's Samantha Stosur 7-6 (7) 7-6 (7) on Wednesday to reach the third round of the Western and Southern Open in Cincinnati.
Back in action for the first time since a semifinal loss to older sister Venus in Montreal last week, Williams faced another familiar foe in Stosur who humbled the American in straight sets in the final of the 2011 US Open.
Williams, who will open defence of her US Open crown at Flushing Meadows in two weeks, has won all four meetings 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 the 22nd ranked Australian 6-0 6-2.
In a tight contest that did not feature a single break by either player, Williams needed nearly two hours to advance to the last 16.
"She was up in both of the breakers, I think it was just a great match, to be honest," said Williams, who has won 61 career titles but 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 off 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 breezed by Russia's Ekaterina Makarova 6-4 6-1.
Ukrainian teenager Elina Svitolina provided the big upset of the day stunning Wimbledon champion Petra Kvitova 6-2 7-6 (2).
Playing her first event since a successful defence of her Baku title last month, the 19-year-old Ukrainian fell behind 3-0 in the second set and twice served for the match at 5-4 and 6-5 but could not close out the match as the third seeded Czech forced a tiebreak.
But Svitolina held her nerve and reeled off five straight points from 2-2 to clinch the upset and advance to the third round in her Cincinnati debut.
"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."