Defending champ Serena Williams reaches quarter-finals

Williams is gunning for her third US Open title in a row but for the moment she saw her 6-3 6-3 Monday victory over Kaia Kanepi as an important hurdle after failing to get past the fourth round in this year's other grand slams.

"I never thought it would be so exciting," the world number one told reporters with a big smile after her 65-minute victory over the Estonian.

"Yeah! It feels good."

So eager to get back to her accustomed presence late in a slam event, Williams celebrated prematurely in the second set when she jumped up and thrust her arms after 50th-ranked Kanepi netted a backhand on break point that made it 5-3 in her favor.

Top seeded Williams failed to serve out the match, but made amends by breaking Kanepi for the fourth time in the set to claim victory and advance to the last eight at Flushing Meadows.

"I finally made a quarter-final this year!" she shouted to the Arthur Ashe Stadium crowd with arms upraised. "Glad to do it in New York."

Williams moved ahead to a battle of 32-year-olds against Italian 11th seed Flavia Pennetta, a semi-finalist last year who advanced with a 7-5 6-2 win against 29th-seeded Australian Casey Dellacqua.

Williams has had a disappointing spell, eliminated in the fourth round of the Australian Open, a round earlier at Wimbledon and in the second round at Roland Garros.

"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," explained Williams about her grand slam struggles.

"Had a couple nagging injuries that definitely didn't help. But other than that, it was just that," she continued.

"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."

Williams said there was no moment of epiphany. "I just have been trying to see bigger picture kind of thing."

Making the quarters would seem like old hat to Williams, who has won 17 career grand slam singles crowns and is bidding to become the first woman to win three U.S. titles in a row since Chris Evert claimed four straight from 1975-1978.

But she certainly appreciates it now.

"Obviously I don't want this to end," Williams said about her campaign at the season's last grand slam.

"But I'm just happy that I'm able to be performing a little better at the end of the year."

Russian Ekaterina Makarova also moved into the quarter-finals with a 7-6(2) 6-4 win over seventh seed Eugenie Bouchard on Monday as the Canadian wilted under the punishing conditions.

With stifling humidity and on court temperatures hovering near 100 degrees Fahrenheit (38 Celsius) it was another day of survival of the fittest at the year's final grand slam and it was a challenge Bouchard could not meet, her bid for a fourth consecutive grand slam semi-final appearance coming to an end.

A tense opening set played under a blazing sun that went to a tie-break and took 50 minutes to decide appeared to drain the 20-year-old Bouchard.

On serve 3-2 in the second, a distressed Bouchard called for a medical time out, trainers rushing onto the baking Louis Armstrong court to rub her arms and legs with bags of ice while checking her blood pressure.

For a moment it seemed Bouchard would not be able to continue as she covered her face with her hands and wept.

After regaining her composure, Bouchard gathered her resolve and returned to action but was quickly broken.

The battling Canadian, however, refused to throw in the towel, immediately breaking back to get back on serve.

Makarova, however, would step up the pressure and in the end Bouchard simply ran out of steam, the 17th seeded Russian breaking her again at 5-4 and ending the ordeal with a sizzling winner down the line.

Twice Australian Open champion Victoria Azarenka overcame an inspired challenge from qualifier Aleksandra Krunic of Serbia 4-6 6-4 6-4.

Azarenka, who began the year as world number two and slipped to a 16th seed after dealing with foot and knee injuries, took a 3-0 lead in the first set before the 21-year-old Serb, ranked 145th, roared back and claimed the set.

Krunic, on course to graduate college with an economics degree this year, continued to press the Belarussian, coaxing 40 unforced errors in the two-hour 19-minute match but in the end Azarenka's power and experience proved the difference.

It ended an enthralling run by the diminutive Serb, who won three qualifying matches, beat 27th-seeded American Madison Keys in the second round and third-seeded Wimbledon champion Petra Kvitova of the Czech Republic in the third round.

Azarenka, the U.S. Open runner-up to Serena Williams the last two years who withdrew from seven tournaments this year due to injuries, will play Russian 17th seed Ekaterina Makarova, a 7-6 6-4 winner over a heat-stricken Eugenie Bouchard of Canada.

"Aleksandra played some amazing tennis. She has a great future," Azarenka told the Arthur Ashe Stadium crowd. "I think it's never easy to play someone you don't know. I just tried to stay positive and fight."