Qualifier Aleksandra Krunic upset Wimbledon champion Petra Kvitova in the third round while Serena Williams advances to the fourth round of the U.S. Open in Flushing Meadows on Saturday.

Krunic looked like she was the defending champion as she stayed calm all throughout the match against third-seeded Kvitova, 6-4, 6-4.

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The 21-year-old Krunic delivered only three aces and 17 winners compared to the five services aces and 33 winners of Kvitova but she produced those at the proper time while breaking the Romanian in the match that lasted an hour and 37 minutes.

The key to Krunic's success was not putting pressure on herself, "I tried to put pressure off my shoulders, you know, because usually last couple of years I'm the one who is putting the pressure on myself," said Krunic.

The 5-ft-5 ½ Krunic returned most of the 6-footers' first serves that put more pressure on the two-time Wimbledon champ.

"I think she played really unbelievable tennis and she put a lot of balls back," Kvitova said of Krunic. "She played really great tennis today."

Krunic advances to the fourth round for the first time in her career and will meet last year's runner-up Victoria Azarenka of Serbia.

World No. 1 Williams continued her quest for a threepeat as she dismantled fellow American Varvara Lepchenko in two straight sets, 6-3, 6-3.

The 17-time Grand Slam champion converted three service aces and 14 winners to advance to the fourth round against surprise entry Kaia Kanepi of Estonia who earlier defeated 15th seed Carla Suarez Navarro of Spain.

On Friday second seed Simona Halep was upset by Croatian qualifier Mirjana Lucic-Baroni to earn herself a fourth round match against Italian Sara Errani who defeated Venus Williams in a three-setter.

This is the 32-year-old second best performance in a major tournament, having advanced to the semifinals of Wimbledon in 1999.

TagsVenus Williams, Sara Errani, Kaia Kanepi