Caroline Wozniacki cited a sound defensive strategy for her victory over Maria Sharapova on Monday as she proved her credentials as top seed at the US Open.

Wozniacki was a 6-3 6-4 winner over the 14th seed on Arthur Ashe Stadium to set up a quarter-final against unseeded Slovak Dominika Cibulkova.

The Dane's consistent defensive play saw Sharapova make 36 unforced errors in the fourth-round clash.

"My strategy was definitely I knew that I had to be ready for the first ball. I knew she was going to try to attack me from the beginning," Wozniacki said.

"I knew she was trying to be aggressive. For me it was important to keep as many balls in the court but still try to move her around and try to dictate, as well."

Wozniacki said she 'enjoyed the rallies' that took place between the pair and was pleased to reach the quarter-final stage as she attempts to win her first Grand Slam.

Cibulkova knocked out 11th seed Svetlana Kuznetsova 7-5 7-6 (7-4) in a battle that lasted over two hours.

Cibulkova, ranked 45th in the world, only won four more points than her opponent over the course of the match but won the points that mattered as she converted half her break point opportunities.

"I was really pumped for this," the 21-year-old said.

"I know my game and I know myself, so I knew I could make it; I could beat everybody in this tournament. You know, I was just waiting for a moment, and I'm happy it's here."

Estonia's Kaia Kanepi surprised 2009 semi-finalist Yanina Wickmayer with a comeback win.

The 25-year-old Kanepi failed to win a game in the first set, but bounced back strongly to prevail 0-6 7-6 (7-2) 6-1.

Wickmayer was largely responsible for her own demise and, after making just five unforced errors in the first set, made 37 over the course of the final two.

It is the first time Kanepi has been beyond the third round at Flushing Meadows, and the result equals her best showing at a Grand Slam.

Kanepi will now face seventh-seeded Vera Zvonareva after the Russian eased past Germany's Andrea Petkovic 6-1 6-2.

The 26-year-old, a finalist at Wimbledon this year, took just one hour and seven minutes to beat Petkovic and advance to her first US Open quarter-final.