Serena Williams made it a hat-trick of victories over fellow Americans to move into the fourth round of the US Open.

The world number one had a much tougher time of it against Varvara Lepchenko than she had against either Taylor Townsend or Vania King but was ultimately a convincing 6-3 6-3 winner in breezy conditions.

Lepchenko, who is of Uzbek descent, had not won more than two games in a set against Williams in their two previous meetings.

She was also looking to become only the second left-handed player after Ekaterina Makarova to beat Williams at a grand slam.

The world number one has shown tunnel vision so far this tournament after her slam wobbles this season and quickly moved into a 3-1 lead.

Lepchenko fought back to level with some terrific play but could not prevent Williams pulling away to take the opening set.

The second was a real battle but Williams won the points that mattered, holding off break points in the fifth game and then forging ahead for 4-3.

That proved the crucial stage of the match and Williams clinched victory after an hour and 35 minutes when Lepchenko netted a backhand.

The two-time defending champion next plays Estonian Kaia Kanepi, who upset 15th seed Carla Suarez Navarro 7-5 6-0.

Wimbledon champion Petra Kvitova became the latest big-name casualty in the women's draw  with a shock third-round loss to qualifier Aleksandra Krunic.

Krunic recorded a 6-4 6-4 over Kvitova, who has never been beyond the fourth round at Flushing Meadows.

The 21-year-old Serbian, ranked 145th, had never won a grand slam match before this tournament, but followed up her upset of Madison Keys with an even bigger scalp.

Kvitova, who had her right thigh heavily strapped, looked to have turned things around when battled back from 4-2 down to 4-4 in the second set, but Krunic did an exceptional job at making the Czech hit one more ball.

Krunic also came up with some terrific shots of her own, showing remarkable composure for a player who had never taken a set off a top-30 player prior to this tournament.

When Kvitova sent her final forehand wide, the Serbian fell to the ground in celebration.

There are now only five of the top 10 women's seeds left in the tournament, with Kvitova joining Simona Halep, Agnieszka Radwanska, Angelique Kerber and Ana Ivanovic in falling early.

Seventh seed Eugenie Bouchard survived another scrap to beat Barbora Zahlavova Strycova 6-2 6-7 (2/7) 6-4.

In her first night session match against Sorana Cirstea two days ago, Bouchard was pushed all the way and it was no different this time.

There is no doubting the mental toughness of the 20-year-old Canadian, though, and she showed it once again despite looking in big trouble in the third set.

Zahlavova Strycova led 30-0 on her serve at 4-4 having just broken Bouchard but she tightened up and paid for it dearly.
Bouchard, who next plays 17th seed Ekaterina Makarova, said: "I feel like I pulled myself together at the very, very end tonight.

"She got a lot of balls back. She made me work for it. At the same time she would occasionally come to the net, mix things up.

"It was the kind of variety I haven't seen much of. Generally most opponents don't do that so it was a little different.

"But I'm just proud at least that I could battle. Even if it's a little bit ugly or I don't feel great, I can still pull it out, which made me happy."