While Rafael Nadal has been grinding down opponents on every surface this year with superior stamina, the third-ranked Djokovic said his own body has some catching up to do.
"Physical strength is something I've been trying to focus on lately," the 21-year-old Serb told The Associated Press in an interview at the Madrid Masters. "I'm aware that this has been one of my issues in that I haven't been recovering well from long matches and long tournaments."
Djokovic won his first Grand Slam tournament this year at the Australian Open, and is hoping for more major titles next year. But he said his tall, lanky frame uses up a lot of energy when running on the court, and puts him at a disadvantage against players like the muscular Nadal and the smooth Roger Federer.
"My game is based on the base line, so I'm running all over the court, stretching, with quick moves, putting a lot of pressure on the body," he said.
With Federer, in contrast, "it's just beautiful to watch the way he plays, the way everything goes with ease."
Djokovic, who was eliminated from the Madrid tournament by Ivo Karlovic on Thursday, said his energy-draining style of play is one of the reasons he has struggled with injuries lately.
"I don't like how I have got a reputation as a guy who asked for medical time-outs to intimidate opponents," he said. "It's not me trying to provoke my opponent, it's me trying to win."
That issue came to the fore at this year's U.S. Open. Djokovic took offense when Andy Roddick had taken him to task over a series of medical issues earlier in the tournament. That did not sit well with the Flushing Meadows crowd, where Djokovic had earlier been a fan favorite because of his impressions of players like Nadal and Maria Sharapova.
"One of the worst moments of my career was when I gave an interview after the Roddick match on the center court and 20,000 people whistled at me," he said.
But while his physical abilities may be lacking, Djokovic said he has developed the mental strength to deal with such setbacks.
"The mental ability that I have at the moment is one of my advantages," he said. "What divides top players from the rest is mental calmness and an ability to cope with pressure in certain moments. That's why you see Rafa, Roger, myself and a couple of other guys at the top. ... If you are mentally able to play the right shots at the right time, then your place is at the top. That's the key of this game."
Djokovic attributed some of his toughness to having grown up in war-torn Serbia. Along with Jelena Jankovic and Ana Ivanovic, he has helped make tennis one of the most popular sports in his country, and is now trying to bring an ATP Tour tournament to Belgrade. That could happen as early as next year, after his family bought the license to the ABM Amro Open.
"We had two wars and lots of economic problems, the image of Serbia was not good. Coming from there made it difficult for me but it made me stronger," Djokovic said. "It will be good to see top level tennis there after so much unrest."
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