US Open: Novak Djokovic beats Del Potro to reach semi-finals

Defending champion Novak Djokovic reached the US Open semi-finals for the sixth consecutive year with a straight-sets win over Juan Martin del Potro.

The number two seed beat the Argentine 6-2 7-6 (7-3) 6-4 in a match more even than the scoreline suggests.

Djokovic, who has not lost a set in the tournament, was pushed hard before a second set lasting almost an hour and a half was settled on a tie-break.

The Serb, 25, will meet Spain's David Ferrer for a place in the final.

Del Potro was full of praise for Djokovic, saying: "He's playing better and better every match. He's the favourite to win."

Although the Serb was not at his best against an opponent whose power caused him considerable problems, Djokovic's superior athleticism and range of stroke eventually told.

A forehand down the line produced the error that brought him victory in three hours and six minutes - an hour and 20 minutes longer than he had been held up in any of the previous four rounds.

"I love playing night sessions," said Djokovic, who hit 43 winners to Del Potro's 25. "We played some incredible rallies and some incredible points.

"I was lucky in the second set to come out with a two-set lead. Even though it was a straight-sets win, it was closer than the score indicated."

Djokovic carried the form that had seen him lose just 20 games in the tournament into this contest, dominating the first set courtesy of two breaks of serve.

However, his 23-year-old opponent - the 2009 US Open champion - responded by breaking Djokovic during a run of 10 straight points to start the second set.

Forced into uncharacteristic errors by Del Potro's booming forehand, Djokovic spent much of the set on the defensive, but his movement and resourcefulness kept him in touch and allowed him to level at 5-5.

Del Potro was forced to save three set points on his own serve - in a pulsating game lasting 17 minutes - and take the second set to a tie-break, which Djokovic won despite losing the first two points.

An ill-advised backhand drop-shot cost Del Potro his serve at the start of the third set, gifting Djokovic an advantage he never looked like relinquishing.

"I had to win that second set," said Del Potro. "In the tie-break he was a little lucky on some points. But he deserved it - he played much better than me for the rest of the match. He was too much for me."

Djokovic will play Ferrer on Saturday, with the victor meeting the winner of the Andy Murray-Tomas Berdych clash in the final.