(Reuters) - A tired sounding Novak Djokovic says he will need to draw on all his mental and physical strength if he is to crown another stellar season with the ATP World Tour Finals title.

The Serb heads into his opening match with Frenchman Jo-Wilfried Tsonga on Monday safe in the knowledge that he will end the year as the world number one for the second consecutive year.

Whether he can add a sixth title to his 2012 resume, which includes the Australian Open, will depend on how his body has recovered since he was knocked out of the Paris Masters by American Sam Querrey on Wednesday.

He admitted after the match he began to struggle physically during the second set, having won the first.

"I want to give my best and at this moment I'm trying to be as optimistic as possible," Djokovic, winner of the season-ending showpiece when it was staged in Shanghai in 2008, told a Sunday news conference at London's 02 Arena which is hosting the event for the fourth year running.

"It's expected to not always on your top form and especially at this time of year the effects of the long season can influence you physically and mentally also, but as I said, this is a very important tournament.

"I definitely want to do well, find the mental and physical strength to perform my best and we'll see if that best will be good enough."

As well as a weary body, reports from Serbia have said the 25-year-old's father was ill, an issue Djokovic was not keen to elaborate on.

"It's not the right moment for me to be talking about these things right, I just want to talk about the tennis tournament."

ENDING ON A HIGH

Regardless of his result in London's docklands, where he has also been drawn to face great rival Andy Murray and Czech Tomas Berdych, he will finish the season as top dog in men's tennis.

He lost the tag to Swiss Roger Federer in July, but will take it back this week in a season that has seen him pocket more than $8 million in prize money.

"It means a lot, for all of us from our team, we are very proud of what we've achieved in 2012," he said of reclaiming top spot. "It's definitely not easy ending the year as number one considering the competition that we had this year, and I'm very happy and very proud and this can only serve as a great confidence boost for me for upcoming years."

While much has been made of his fierce battles with London-based Scot Murray, which has seen Djokovic prevail in the Australian Open semi-finals in a match lasting close to five hours before Murray took revenge at the same stage of the London Olympics and then in the remarkable five-set U.S. Open final, he preferred to dwell on season where four different players - himself, Murray, Federer and injured Rafael Nadal - won a major.

"For me, it's great to be part of that rivalry, to be part of this era between four of us really good players, and we are trying to bring the sport to another level, and it's really nice to see these rivalries are evolving, these rivalries are getting stronger, and more competitive," he said.

TSONGA CHALLENGE

Djokovic said the round-robin format of the tournament was a nice change from the usual knockout tennis players encountered, as you could still make the semi-finals having lost a match.

He hopes to get off to a good start against world number seven Tsonga however, who he beat in the final of the China Open last month.

"We always have good battles, tough battles, he is a big server and he is the kind of player that feeds off the energy of the crowd," he said.

"He played finals here last year, it's going to be very difficult for both of us."

(Reporting by Josh Reich; editing by Martyn Herman)