With temperatures pushing past 30 degrees Celsius at Flushing Meadows, the Canadian - who has reached at least the last four of every Grand Slam this year - was out of sorts.

The 20-year-old, who came into the tournament seeded seventh, lost 7-6, 6-4 to Russian 17th seed Ekaterina Makarova.

Bouchard said afterwards: "I was feeling very light-headed and dizzy on the court, just seeing things a little blurry.

"Feeling well physically on the court is very important to me.

"I just generally didn't feel good.

"I think it developed as the match went on.

"In the middle of the first set I started not feeling great.

"But I have had a few late, tough matches here, and I don't think I fully recovered from those."

The beaten Wimbledon finalist needed three sets to beat Romanian Sorana Cirstea in their second round encounter on Thursday (August 28)

Belarusian Victoria Azarenka, the beaten finalist in the last two US Open women's singles finals, faced the prospect of a similar fate to Bouchard before rescuing victory.

The 16th seed dropped the first set against Serbian Aleksandra Krunic 6-4, before winning the next two by the same scoreline to progress into the quarter-finals.

On a day where the top seeds showed their prowess, Serena Williams of the Untied States powered into the last eight of a Grand Slam for the first time this year.

She dispatched Estonia's Kaia Kanepi 6-3, 6-3 in 53 minutes.

Novak Djokovic remains on course to add to his 2011 US Open title ©Getty Images



Men's top seed Novak Djokovic was equally untroubled.

The Serbian beat German Philipp Kohlschreiber in straight sets, 6-1, 7-5, 6-4 - reaching the quarter-final of a Grand Slam for the 22nd consecutive time.

Great Britain's Andy Murray faced what looked on paper like a tricky match against Frenchman Jo-Wilfried Tsonga, but showed some of his best form of the year to win comfortably 7-5, 7-5, 6-4.

Australian Open champion Stan Wawrinka is also through to the last eight.

The Swiss beat Spain's Tommy Robredo 7-5, 4-6, 7-6, 6-2.

Tomorrow sees Wawrinka's countryman, Roger Federer, in fourth round action against Roberto Bautista Agut of Spain, while Maria Sharapova's conqueror, Dane Caroline Wozniacki, will be in quarter-final action against Sara Errani of Italy.