Caroline Wozniacki, who lost in the final last year, is looking the part of the favourite this time around. (Kathy Willens/Associated Press)

Top women's seed Caroline Wozniacki met her first challenge at the U.S. Open on Monday and responded by eliminating former tournament champion Maria Sharapova in straight sets.

The Dane won by scores of 6-3, 6-4 at Flushing Meadows, N.Y. Sharapova matched Wozniacki in terms of power, but was inconsistent on her serve and hit more than three times as many unforced errors.

Sharapova, born in Russia but a U.S. resident for several years, has made the quarter-finals at a major just once since winning the 2008 Australian Open. She has played better of late after dealing with shoulder injuries but still can't produce consistent results.

She appeared ready to take Wozniacki into deep waters early on but completely lost focus on her serve in the fourth game, offering two of her nine double faults on the day.

Sharapova persevered in the seventh game, finally converting on her sixth break-point chance to get within 4-3. She fought off three set points in the ninth game, but could not prevent Wozniacki from taking the set.

The No. 17 seed was at her strongest early in the second, playing on even terms through six games.

The fifth game was one of the few easy ones of the set, with Sharapova firing an ace to go up 3-2. She then put Wozniacki down 0-30 in the next game, but the top seed roared back to win four points in a row.

Sharapova smacked a forehand into the net and then hit her ninth double fault to go down 4-3, the beginning of the end.

Wozniacki, who overcame a fall while running backwards earlier in the set, hit a volley at the net to set up match point and smacked a two-handed backhander to end the matter.

Wozniacki committed just 10 unforced errors to 35 for her opponent as she looks to follow up on consecutive wins in Montreal and New Haven, Conn., with the first Grand Slam of her career. She lost to Kim Clijsters of Belgium in last year's Open final.

"I knew it wasn't going to be easy," Wozniacki said. "I got a lot of experience from last year, so it definitely helped me today."

Wozniacki lost just three total games in her first three matches of the tournament. The 20-year-old will next play Dominika Cibulkova of Slovakia.

Cibulkova, Kaia Kanepi and Gael Monfils are all on to the U.S. Open quarter-finals for the first time in their careers after posting wins. While they are newcomers to that stage, Novak Djokovic is an old hand, reaching the final eight for the fourth straight year at the Open.

Cibulkova eliminated former Open champion Svetlana Kuznetsova, rebounding after squandering two match points to defeat the No. 11 Russian 7-5, 7-6 (3).

Kuznetsova won the U.S. Open in 2004.

"I think she plays good, but my level is higher, and I have to win these matches," Kuznetsova said.

Cibulkova was ranked a career-high 12th just over one year ago, but has dropped to 45th heading into the final Slam of the season.

She lost seven of her last nine matches heading into the Open, including a loss in New Haven to Wozniacki.

Monfils prevailed in straight sets over fellow Frenchman Richard Gasquet, reaching the quarter-finals at a Slam other than the French Open for the first time in his career.

The No. 17 seed won by scores of 6-4, 7-5, 7-5, rebounding with a rally in the final set when it appeared Gasquet would take the match into a fourth set.

Monfils will have to beat Djokovic for the first time in five career meetings.

Djokovic battered Mardy Fish 6-4, 6-3, 6-1. Fish's defeat leaves Sam Querrey the lone American man left in the singles draw.

"I'm playing with a lot of confidence," Djokovic said. "It's definitely great to raise the level of performance toward the end of the tournament. It's been a great couple years for me in New York. Hopefully, I can go on."

The Serbian has now made it to at least the quarter-finals in all but two of his last 15 Grand Slam tournaments. After losing in the 2007 U.S. Open final to Roger Federer, Djokovic has bowed out in the semis the last two years.

Monday's night session saw No. 5 seed Robin Soderling advance to the quarter-finals, serving 15 aces in a 4-6, 6-3, 6-2, 6-3 victory over 21st-seed Albert Montanes. Soderling's next challenge is a familiar one, five-time U.S. Open champion Roger Federer.

The Swiss star beat 13th-seed Jurgen Melzer of Austria 6-3, 7-6 (4), 6-3 to reach his 26th consecutive Grand Slam quarter-final round.

It was Soderling who ended Federer's streak of 23 consecutive Grand Slam semifinals by upsetting him in this year's French Open quarterfinals.

"He's always been a dangerous player," Federer said in an on-court interview. "This is obviously a tough draw for me in the quarters, playing Robin."

Estonia's Kanepi pulled off a surprising result against last year's Open semifinalist, Yanina Wickmayer of Belgium. Kanepi dropped the first eight games but went on to win 0-6, 7-6, 6-1.

Kanepi, 25, set herself up with an upset win on the weekend over Jelena Jankovic.

Kanepi will next face No. 7 Vera Zvonareva of Russia, who overwhelmed 38th-ranked Andrea Petkovic of Germany 6-1, 6-2 Monday night to reach the U.S. Open quarterfinals for the first time.

With files from The Associated Press