Kei Nishikori overcame top seed Novak Djokovic in four sets to reach the final of the US Open at Flushing Meadows.

The tenth seed from Japan produced a quite superb display to record a 6-4 1-6 7-6 (7/4) 6-3 triumph, becoming the first Asian man to reach the final of a grand slam tournament.

Despite having to come through gruelling five-setters in both previous rounds, Nishikori displayed tremendous endurance on a scorching day in New York as both players battled the heat as well as each other.

Djokovic - who was bidding to reach a fifth consecutive final at the event - looked to have asserted himself on the match as he raced through the second set after dropping the first, but the seven-time grand slam champion was ultimately undone by the ferocity and consistency of Nishikori's groundstrokes as the 24-year-old clinched a career victory.

"I don't know what's going on," a delighted Nishikori stated immediately after his win. "I was a little bit tight, especially with my first semi-final in a grand slam. It's just an amazing feeling, beating the number one player and playing much better than the last couple of matches.

"I'm so happy. It was tough conditions but I guess I love to play long matches and I hope I can recover well for the final.

If Nishikori was tired or nervous going into his first semi-final, he did not show it, more than matching Djokovic from the back of the court.

An early break came and went but Nishikori was the one dictating a lot of the points and he placed a forehand return onto the outside of the line to move ahead again at 4-3, this time making the advantage count as he served out for the set.

Djokovic hits back

But he could not stem the Djokovic tide in the second set, the 2011 champion stepping up the aggression and winning five games in a row to level the contest.

Nishikori was then forced to dig deep to hold serve in the third game of the third set, but his endeavour was rewarded as he combined a backhand pass with forehand winner to break the Djokovic serve in the eighth game.

That left him serving for the set but, having been so cool, Nishikori wobbled, double-faulting on break point.

Djokovic roared as he held to make it 5-5 but the world number one was strangely passive and paid for it in a lacklustre tie-break, consecutive forehand errors seeing Nishikori move ahead in sets once again.

Clearly fatiguing, when Djokovic handed over his serve meekly at the start of the third set, the alarm bells really started ringing and, although he had three chances to break straight back, Nishikori saved them all.

Djokovic certainly appeared to be struggling more with the heat and he never really looked like clawing his way back as he has done from similar situations on so many occasions before.

Nishikori kept attacking and, with Djokovic serving at 3-5, brought up two match points. The top seed saved the first but went long on the second, allowing a delighted and slightly stunned Nishikori to celebrate the biggest win of his life.