Serena Williams breezed into the third round of the US Open and saw the draw open up around her.

The two-time defending champion is perfectly capable of bulldozing her way through the toughest of draws but cannot now meet a top-10 rival until the semi-finals after Ana Ivanovic lost to Karolina Pliskova.

Sam Stosur, who stunned Williams in the 2011 final, could have been a fourth-round opponent but she fell to Kaia Kanepi in a deciding tie-break after missing two match points.

That leaves last year's semi-finalist Flavia Pennetta, seeded 11th, as the highest-ranked player in Williams' path to the last four.

The world number one has been in no mood to mess around this tournament and allowed compatriot Vania King only one game in a 6-1 6-0 victory in round two on Thursday.

Williams did not find the windy conditions easy to cope with but was ruthless in her execution on the big points.

The 32-year-old said: "It's so hard to play in the wind. I'm really happy to get through a solid match with the conditions today."

Williams plays her third American opponent in a row in the next round, taking on Varvara Lepchenko.

Ivanovic became the second top-10 player to crash out of the tournament, joining Agnieszka Radwanska in making a shock early exit.

The eighth seed went into the tournament with high hopes of marking her best season since she won the French Open in 2008 with a good run in New York.

But she found world number 42 Pliskova in fine form and succumbed to a 7-5 6-4 defeat, with the 22-year-old Czech reaching the third round at a grand slam for the first time in her career.

For all her success this year, winning three titles, deep runs at slams have continued to elude Ivanovic.

The honourable exception was the Australian Open, where she defeated Williams to reach the quarter-finals, but she then went on to lose in the third round at both the French Open and Wimbledon.

Ivanovic credits her resurgence this season with learning to relax but admits she still puts too much pressure on herself at the slams.

The Serbian said: "I had great lead-ups to every grand slam. I played a lot of matches and won lots of matches. And big matches, as well.

"At the grand slams I just haven't performed that well. It's something that I'm definitely going to work on and see where I can improve, because that's definitely the big aim for me for next season.

"I do put a lot of expectations on myself. I tried to over-analyse and over-think instead of just playing the game. It's a work in progress. But I really think I have the potential to do well."

The other seed to fall in Williams' quarter was Russian Anastasia Pavlyuchenkova, who was beaten by 21-year-old American Nicole Gibbs.

Wimbledon champion Petra Kvitova won an all Czech battle with Petra Cetkovska 6-4 6-2 to keep her good run going.

Victoria Azarenka looked much sharper than in her first-round match, winning 12 of the last 14 games in a 6-3 6-2 victory over America's Christina McHale.

Azarenka, the beaten finalist at Flushing Meadows the last two years, has had a nightmare season with injury and is seeded only 16th this time.

She needed three sets to battle past Misaki Doi in round one and lost the first three games to McHale before recovering in impressive fashion.

There was disappointment, though, for American 27th seed Madison Keys, who slumped to a 7-6 (7/4) 2-6 7-5 defeat by qualifier Aleksandra Krunic.