NEW YORK (Reuters) - Novak Djokovic, dogged by nagging questions about his stamina and guts when pressed in big matches and high temperatures, showed fortitude against an old friend at the U.S. Open on Tuesday.
The 23-year-old Serb dug deep to come back after trailing two sets to one and down a break in the fourth set to overtake his Davis Cup team mate Viktor Troicki 6-3 3-6 2-6 7-5 6-3 in the first round on Arthur Ashe center court.
"I've been in those situations before, played a lot of long matches in very difficult conditions, feeling very exhausted," 2008 Australian Open winner Djokovic told reporters.
"You kind of start panicking a little bit when you don't feel great physically. Then your opponent takes the advantage. And it's not easy. Definitely those moments are very challenging for an athlete."
Djokovic absorbed heavy criticism when he retired during his Melbourne defense in 2009 because of heat illness in the quarter-finals and then withdrew from Wimbledon complaining of a blister on his toe.
This year Djokovic, who has struggled with allergies and sinus problems, withdrew from the Serbian Open quarters because of breathing problems. Three weeks ago he called for medical treatment while playing under intense heat in Toronto.
But with on-court temperatures reaching 109 Fahrenheit (43 Celsius), Djokovic refused to quit against 47th-ranked Troicki, a friend and foe since they were kids.
Over the years, the two have shared many victories together, joining forces this year to carry Serbia to their first Davis Cup world group semi-finals against the Czech Republic in September.
"We grew up together playing in juniors. We're practicing together for a long time. We know each other since we were I think nine years old," said Djokovic, U.S. Open runner-up in 2007 and semi-finalist the last two years.
"My first tournament in my life that I've played, it was under-10, I won my first round and then I played him second round. He destroyed me."
Djokovic, winner of the previous four matches against the 24-year-old Troicki, got another measure of revenge against him on center court Tuesday and dispelled some doubts about his distaste for adversity.
Reporting by Larry Fine; Editing by Frank Pingue