Serena Williams
Serena Williams reacts during the match against Samantha Stosur on day three.Reuters

World number 1 Serena Williams defeated 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.

The American was back in action after she lost to older sister Venus Williams in the semifinals of Rogers Cup last week. Williams had a tough battle against Stosur, who had defeated her in straight sets in the 2011 US Open final.

Williams, who will defend her US open title in two weeks, has now won all four encounters against the Australian since 2011 US Open. However, the 17-time Grand Slam winner had to fight harder than she did in Montreal last week, where she successfully got past her 22nd ranked opponent 6-0, 6-2.

Williams took nearly two hours to advance to the third round. The game did not feature any breaks by either player.

The American has lifted 61 career titles but has never won the Cincinnati trophy. She said that the match was really intense against because of Stosur's great serves.

"She was up in both of the breakers, I think it was just a great match, to be honest," Williams said. "She served unbelievable, and I was like, I can't lose serve because she's just serving great."

Meanwhile, fourth seed Agnieszka Radwanska, who sealed Rogers Cup in Montreal on Sunday, cruised to a 6-2, 6-2 win over Japan's Kurumi Nara, while German sixth seed Angelique Kerber defeated Russia's Ekaterina Makarova 6-4, 6-1.

However, Wimbledon champion Petra Kvitova was thrashed 6-2, 7-6 (2) by Ukrainian teenager Elina Svitolina to give the biggest upset of the day.

"I have to say number one I'm disappointed how I played today, for sure," Kvitova pointed out. "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."

In other results, Serbians Jelena Jankovic and Ana Ivanovic both got through to the third round.