Novak Djokovic kept alive his hopes of ending Roger Federer's dominance at the US Open in New York as the third-seeded Serb edged past Radek Stepanek of the Czech Republic.

In an epic, four-hour and 44-minute battle of nerves, the man who beat both Rafael Nadal and Federer en route to winning the ATP Masters Series in Montreal three weeks ago needed a fifth-set tie-break to finally put daylight between himself and the world number 35.

Djokovic won 6-7 (4-7) 7-6 (7-5) 5-7 7-5 7-6 (7-2), sinking to his knees after beating his adversary with a deft half-volley at the end of a tense final rally.

Fired-up Stepanek had used every trick he could to unsettle the 20-year-old, staring down the Serb from across the net after points and playing to the crowd at Armstrong Stadium with dramatic celebrations of big points.

The Czech took the first set on a tie-break before Djokovic replied in kind and there was little to separate the eastern Europeans in the third until Stepanek broke Djokovic for a 6-5 lead to serve for the set.

He showed signs of nerves when he double faulted for the first time in the match to cede a 15-30 lead to Djokovic but soon reverted to type and took the set.

Djokovic, who had had his legs rubbed down during the third set, needed a medical time-out during a fourth-set changeover having taken a 2-1 lead.

Djokovic's moment came at 6-5, when he finally broke Stepanek to level the match at two sets all.

The final set went with serve to take the match into a deciding tie-break and Stepanek's resolve finally crumbled. Djokovic quickly gained control to win the breaker 7-2.