Novak Djokovic of Serbia celebrates after defeating Juan Martin Del Potro of Argentina during their men's singles quarterfinals match at the US Open tennis tournament in New York, September 6, 2012.??[Photo/Agencies]

NEW YORK - Novak Djokovic was at his brilliant best as he moved a step closer to defending his US Open title with a 6-2 7-6 6-4 victory over Juan Martin del Potro to reach the semi-finals at Flushing Meadows on Thursday.

The second-seeded Serb stamped his authority on the quarter-final when he edged a titanic second set against the 6ft-6in (1.98 m) Argentine, who won here in 2009.

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

It was tennis of the highest order as the pair pushed each other all around the court but Djokovic emerged with the set after taking the tiebreak 7-3 against the seventh seed, who more than once draped himself over the net in exhaustion.

"Even though it was a straight-sets 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 major title.

"I was lucky in the second set to get out with a two-set advantage. We played some incredible rallies, some incredible points."

Djokovic excelled from the service line, getting in a remarkable 84 percent of his first serves and allowing Del Potro to convert only one of three break points in the finale to the 11th day of the tournament.

The Serb also produced some incredible service returns, often leaving the Argentine shaking his head and waving his arms in frustration over sharply angled shots that eluded him.

"He was too much for me," Del Potro said. "I think he's the favourite to win this tournament. I wish him the best."

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 in a marathon four-and-a-half-hour tussle.

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

After dropping the first set, Del Potro drew first blood in the second, claiming a service break in the opening game and maintaining his lead until Djokovic broke back in the 10th game to level at 5-5.

The Serb held serve for 6-5 and put everything he had into finishing off Del Potro, who refused to buckle under the pressure of the line-brushing, groundstoke blasts that kept coming off Djokovic's racket.

The game went to eight deuces with Del Potro extending the set to a tiebreaker when Djokovic sent a service return long on the 22nd point of the duel.

Djokovic maintained his intensity through the tiebreak, running off the last four points after they were knotted at 3-3.

Del Potro sagged at the start of the third set, losing his serve in the opening game on a weak backhand that barely dented the net and was never about to recover.

With the match winding down, Del Potro was able to share a light-hearted moment with the Arthur Ashe Stadium crowd when he raced to his backhand side to hit a winner and jumped up on a barrier in front of the stands to soak up the cheers with arms outstretched.

"It's amazing for me to share with the crowd this kind of moment. I really enjoyed playing in this stadium. I had my big memories here in this tournament and I would like to say thank you for these things."