US Open 2014: Eugenie Bouchard becomes the latest name to crash out as Serena Williams stands tall as the last top seed remaining

Bouchard lost to Ekaterina Makarova while Williams overcame Kaia Kanepi to reach the quarter-finals as the last remaining player in the top eight seeds

Serena Williams celebrated reaching her first grand slam quarter-final of the year at the US Open - and finds herself as the only top-eight player to have made it that far.

Seventh seed Eugenie Bouchard joined the exodus of the leading players, struggling badly in the hot and humid conditions on her way to a 7-6 (7/2) 6-4 defeat by Russia's Ekaterina Makarova.

Bouchard, who had made at least the semi-finals at the other three grand slams this year, had her blood pressure taken after five games of the second set and was in tears, but was at least able to finish the match.

The 20-year-old Canadian said: "I was feeling very light-headed and dizzy on the court, seeing things a little blurry. I have had a few late, tough matches here and I don't think I fully recovered from those.

"I'm always disappointed to lose, and especially knowing that I couldn't give everything. But I didn't have the highest expectations from myself for this tournament.

"Since Wimbledon it's been a little bit of a struggle with nagging injuries. I have really cut down on practice time. That affects you in a match.

"I'm not concerned at all, but with all that and with not feeling great in my tennis, I still battled to the second week of a slam. So there's positives."

Bouchard receives medical attention during her defeat to Makarova

There have been none of the wobbles from Williams that led to unexpected defeats in Melbourne, Paris and at Wimbledon.

She has not lost more than three games in any set and was a relatively comfortable 6-3 6-3 winner over Estonia's Kaia Kanepi in the fourth round on Monday.

Williams has looked totally focused on the job in hand this tournament and did not lose a point in her first three service games.

Kanepi, another big hitter, kept pace with her initially but once Williams made the breakthrough for 5-3, the first set was gone.

Having broken again in the opening game of the second, there was an embarrassing moment for the two-time defending champion when she completely fluffed a smash and dropped her serve to love.

But it was soon forgotten, as was the eighth game when she served for the match and was broken, with Williams clinching victory on Kanepi's serve

The American raised her hands in the air during her on-court interview, and said triumphantly: "I finally made a quarter-final this year!"

"I never thought it would be so exciting," Williams said later. "It feels good. Obviously I don't want this to end. But I'm just happy that I'm able to be performing a little better at the end of the year.

"I'm a perfectionist, I always want to be the best, do the best. I think I put a lot of pressure on myself to do well, particularly in the slams. I had a couple of nagging injuries that definitely didn't help.

"Now I'm more relaxed. I feel like I don't have to win any more. I've had a wonderful career. Tennis has given me so much, things I would have never expected in my life. I feel honoured to even be a part of such a wonderful sport."

In the last eight, Williams will face 11th seed Flavia Pennetta, who is having another good run at Flushing Meadows.

The Italian has only made one quarter-final at all the other grand slams combined but is through to her fifth in New York after a 7-5 6-2 victory over Casey Dellacqua.

"Everyone asks the same question," said Pennetta when asked to explain her success. "I don't know. I just like to play here and maybe that's why I play good here."

Pennetta has lost all five previous matches against her fellow 32-year-old Williams but will head into the clash with a positive attitude.

She said: "You just have to play your tennis and do your best. Of course, she's better than me, but if I believe I can beat her, maybe if she doesn't have a good day I can do that.

"If I get on the court and just play and try not to lose 6-0 6-0, I'm going to lose 6-0 6-0."

Victoria Azarenka stayed on course for a third consecutive final appearance with a battling win over qualifier Aleksandra Krunic.

The 21-year-old Serbian has been one of the stars of the tournament, beating both Madison Keys and Petra Kvitova, and for a long time it looked like she might add another huge scalp.

The diminutive Krunic embraced her debut on Arthur Ashe Stadium and was an instant crowd favourite, wowing the fans with her court coverage and powerful hitting.

She won the first set and caused problems for Azarenka throughout but it was the 16th seed who came through 4-6 6-4 6-4 after two hours and 19 minutes to set up a last-eight clash with Makarova.

Azarenka, who has struggled with injury this season, said: "Alexandra played some amazing tennis. She has a great future.

"It's never easy to play somebody you don't know. She stepped it up and I just tried to stay positive and fight and do anything I could to turn the match around."