By Larry Fine

NEW YORK (Reuters) - Serbian third seed Novak Djokovic successfully countered big-hitting American Mardy Fish's service game Monday to reach the U.S. Open quarter-finals.

Djokovic, who survived a five-set struggle in the first round, shifted into top gear for his best performance of the tournament to claim a 6-3 6-4 6-1 victory over the 19th seed.

The 23-year-old Serb, a semi-finalist at the U.S. Open in the past two years and runner-up in 2007, unleashed some formidable returns and used his superior quickness and shot-making skills to turn the fourth-round test into a rout.

Djokovic drilled 30 winners past the sluggish Fish, 28, and even outperformed him from the service line with five aces to just three for the American.

After reaching his sixth successive grand slam quarter-final, Djokovic said he was most pleased with the focus he maintained in Arthur Ashe Stadium.

"Whenever I had the chance I used it," said the Serb, who converted six of eight break points in the 110-minute match.

"When he served his games I was returning really well and I was serving really well, using the court well. Overall I'm very happy with the way I played."

Djokovic will next meet France's Gael Monfils, who defeated compatriot Richard Gasquet in straight sets. Djokovic has won all four of his previous meetings against world number 19 Monfils.

The Serb is gaining momentum as he goes, and is on course for another clash against Swiss master Roger Federer in the semi-finals.

Djokovic was eliminated the last three years at the Open by Federer.

"This gave me a lot of confidence," the Serb said about his dominant play. "It's great to raise the level of performance toward the end of the tournament.

"It's been a great couple of years for me in New York so hopefully I can go on."

(Editing by Steve Ginsburg)