London :Novak Djokovic admits he is struggling to find the energy to take on Rafael Nadal in a match that could end his reign as ATP World Tour Finals champion.
World number three Djokovic cut an exhausted figure as Sweden's Robin Soderling overpowered him in the second set at London's O2 Arena on Wednesday to claim a 7-6 (7/5), 6-1 victory.
The Serb now needs to beat Australian Open champion Nadal on Friday to have any chance of making the semi-finals as he tries to defend the trophy he won for the first time in Shanghai last year.
However, a gruelling 11-month campaign has worn down the 22-year-old to such an extent that he conceded it will be hard to drag his weary limbs into battle one more time.
Asked if he could get ready for the Nadal match, Djokovic said: "I don't know. I will see. I'm very tired. It's just fatigue of the whole year."
In his position as a member of the ATP Player Council, Djokovic has already had to debate whether it would be better for players' fitness to shorten the season.
Although the Serb seems to be suffering more than most from the daunting schedule that the ATP Tour demands of its' top stars, he refused to criticise the sport's bosses.
"I don't know (if the season is too long). I prefer not talking about it now," he said.
"I don't think I played too much. I just played very solid in all the tournaments that I was committed to play. So I played all the tournaments that I had to play."
While Djokovic can't wait for the season to finish, Soderling is having the time of his life in London.
Although he was only the first alternate for the tournament, he arrived in the English capital five days before the first match in case any of the players withdrew and was rewarded when Andy Roddick pulled out injured.
"I knew for a pretty long time that I was going to be at least first alternate here, so I prepared as if I was going to play," he said.
Ending Nadal's four-year reign as French Open champion en route to the final, where he was beaten by Roger Federer, has transformed Soderling's status on the Tour from journeyman to rising star.
The world number nine believes the key to his success has been his ability to control his fiery temper, which had often led him into costly on-court meltdowns when things didn't go his way.
"I try to work a lot on it," he said. "Of course I lost a lot of matches because of my temper. Everyone is always talking about the matches I lost, not the matches I won, because of my temper.
"But I think I have a little bit of a different approach to everything this year compared to the other years.
"I always put a lot of pressure on myself. But I never really cared about the pressure from others.
"I only won two matches so far, but it's against the world number two and three. I don't think I ever done that before back to back.
"Except for the grand slams, this the biggest tournament of the year. It's very good, but there's still more to come."