Everyone loves a funny man, but when he starts to get serious, no one is sure how to react. Novak Djokovic was the jester in New York a year ago and his impersonation of Andy Roddick was one of his best. However, this year he is finding the laughs harder to come by.

Djokovic is through to his second US Open semi-final today, when he faces Roger Federer, the world No2 from Switzerland, in a rerun of their 2007 final, but the audiences are tougher this time. A lot of that may be because the 21-year-old Serb is intense to the point of being fit to burst. When he looks as if he is tired, the crowd think that he is faking it. He cannot win, except that he does.

His 6-2, 6-3, 3-6, 7-6 victory over Roddick in the quarter-finals on Thursday was a case in point. The No6 seed from the United States had been asked before the match to comment on Djokovic's perceived litany of injuries and said: “I've got to feel good, he's got about 16 injuries, back and hip, cramp, bird flu, anthrax, Sars [severe acute respiratory syndrome].” Djokovic did not see the funny side and when he took a medical time-out, those in the crowd on Roddick's side (about 98 per cent) took exception.

Djokovic did not do himself many favours when, in an on-court interview, he baited those who had baited him. “They're already against me because they think I'm faking everything,” he said. You do not do that to New York crowds and expect warm applause.

All of this is a terrible pity because it detracts from Djokovic, the immense player, the fighter, the committed soul who has heard Federer and Rafael Nadal talked of as the great rivalry and wants to remind the world of how good he is. He has said that he is on the point of exhaustion, but he will not give in. There is a touch of the Jimmy Connors about him and it was apt that Connors, who parted company with Roddick this year, should have been in attendance to see his former charge worn down by Djokovic's belligerence.

There remains in American tennis an uneasy sense that Roddick, 26, the 2003 champion and former world No1, has peaked in grand-slam terms, but he was good enough to reach two Wimbledon finals, in 2004 and 2005, and the 2006 US Open final, in which he faced Federer, who was a class above everyone else at the time. “I was playing absolute garbage tennis before I came into this tournament, so I'm happy with the efforts I made to try to come back,” the American said.

If others are too quick to give up on him, Roddick ought never to give up on himself.