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.

The fairytale is over for 15-year-old American Cici Bellis after she was beaten in the second round by Zarina Diyas of Kazakhstan.

The Californian teenager was the name on everyone's lips at Flushing Meadows after her stunning upset of Australian Open finalist Dominika Cibulkova on Tuesday.

Some fans had taken their seats on Court 17 first thing on Thursday morning in anticipation but it was not until after 8pm that Bellis and Diyas had their moment.

Diyas, who is only 20 herself, has risen more than 100 places in the rankings this year, reaching the fourth round at Wimbledon.

She handled a difficult situation well, with every error cheered and every point for her opponent greeted with an explosion of noise.

Bellis, who won her place in the draw by virtue of becoming the US under-18 champion, gave as good as she got and it will be a surprise if she is not playing regularly on this type of stage very soon.

But in the end it was Diyas who was steadier in the deciding set, the Kazakh recovering from being bagelled in the second to win 6-3 0-6 6-2.

Bellis, who will stay in New York to play the junior tournament, said: "This whole experience has been unbelievable, mind-blowing. It's been crazy. It's been the best couple of days of my life.

"(I've heard) people saying that I'm going to be the future of American tennis. That's what I've wanted to be since I was a little kid. I think that definitely makes me want to work really hard and try to become that.

"What surprised me is that I could really stay with these pros. And I think today if I had played a little bit better, it would have been a different result."

Seventh seed Eugenie Bouchard was given a real battle by Romania's Sorana Cirstea in the final match of the night but survived to reach round three.

Cirstea is a former top-30 player who has fallen to 80th in the rankings but she showed the talent she has in a first-round rout of Britain's Heather Watson.

And Cirstea took that form into the clash with Bouchard, her forehand frequently pushing the ultra-aggressive Canadian onto the back foot.

But Bouchard is made of seriously tough stuff and every time Cirstea pegged her back, she edged ahead again.

The Romanian saved a match point at 3-5 in the final set but Bouchard was not to be denied, eventually clinching a 6-2 6-7 (4/7) 6-4 victory.

Her reaction showed what a tough test it had been, and the Canadian said: "I definitely had to battle in the third (set). She was getting really solid in the second. I just believed in myself and told myself to keep fighting."

The 20-year-old has a remarkable streak to maintain, having reached at least the semi-finals at every grand slam this season.