By Larry Fine

NEW YORK (Reuters) - Novak Djokovic took a giant step in his quest to repeat as U.S. Open champion by beating Juan Martin del Potro 6-2 7-6 6-4 to reach the semi-finals at Flushing Meadows on Thursday.

The second-seeded Serb gained the decisive edge in the quarter-final by winning a titanic second-set struggle against the 6-foot-6 Argentine, who won here in 2009.

The set lasted 84 minutes and featured a 17-minute game at 6-5 when 23-year-old Del Potro saved three set points to force a tiebreaker.

It was tennis of the highest order as the players sent each other from corner to corner, and from net to baseline and back again, but Djokovic emerged with the set after taking the tiebreak 7-3 over the seventh seed, who more than once draped himself over the net in exhaustion.

"Even though it was a straight set win, it was much closer than the score indicated. He's a great player," said Australian Open champion Djokovic, 25, who reached his 10th successive grand slam semi-final as he pursues a sixth slam title.

Djokovic will meet fourth-seeded David Ferrer of Spain, who beat eighth seed Janko Tipsarevic of Serbia 6-3 6-7 2-6 6-3 7-6 by taking the fifth-set tiebreaker 7-4.

Saturday's other semi-final will pit Olympic champion Andy Murray of Britain against Tomas Berdych, who beat Roger Federer in the quarter-final.

(Editing by Peter Rutherford)