Second seed Victoria Azarenka kept her title hopes alive at the US Open with another battling performance to defeat Ana Ivanovic and reach the quarter-finals.

Azarenka came into the tournament on the back of a victory over Serena Williams in the Cincinnati Masters final but has struggled to find her best form in New York.

In the third round she lost the first set to France’s Alize Cornet before fighting back, and it was the same story here as she ground out a 4-6 6-3 6-4 victory after two hours and 10 minutes.

The match had been put back a day because of a lengthy rain delay, even though all the other singles matches were completed on schedule. That could put Azarenka at a disadvantage, with her quarter-final opponent Daniela Hantuchova having an day extra to prepare.

Ivanovic had been showing signs of the form that brought her the French Open title in 2008 and took her to world number one but beating the very top players has proved a challenge too far.

With her fearsome forehand firing, the 13th seed took the first set, but Azarenka is one of the game’s great fighters and she levelled the match.

The decider was a tortuous affair at times, with only three service games of 10 held.

Ivanovic saved two match points to get back to 4-5 and then another on her own serve but on Azarenka’s fourth chance the Serb netted a backhand.

Ivanovic has made just one grand slam quarter-final – here last year – since her victory at Roland Garros. Azarenka remains a big favourite to repeat last year’s run to the final, when she lost to Williams, with Hantuchova, Roberta Vinci and Flavia Pennetta the other players left in her half.