London - Roger Federer will start his bid for a seventh title at the season-ending ATP World Tour Finals with a revenge mission against Novak Djokovic.
After one of the worst campaigns of his illustrious career, Federer arrives for the prestigious event at London's O2 Arena looking to finish the year on a high note and he has an opportunity to make an immediate impact when he faces world number two Djokovic on Tuesday.
Federer has tumbled to sixth in the world rankings this year and just barely qualified for the ATP's eight-man showpiece tournament.
With many pundits writing off the 32-year-old as a fading force, Federer would dearly love to prove he can still shine against the sport's big guns by beating Djokovic in his opening match before further Group B clashes against Juan Martin del Potro and Richard Gasquet.
It would also go some way to avenging his loss to the Serb in the semi-finals of the Paris Masters on Saturday and also a straight sets defeat in the final of last year's Tour Finals, which ended his hopes of a third successive title at the O2 Arena.
Federer's decline can be measured by his failure to reach a Grand Slam final this year -- the first time he hasn't appeared in one of the four major finals since 2002.
But the 17-time Grand Slam champion has at least come closer to his peak form over the last two weeks, reaching the final of his hometown event in Basel before losing to del Potro and then pushing Djokovic hard in the last four in Paris.
"It's unusual to go back to back matches against Novak, but I'm always excited about the challenge," Federer said.
"I have had a good couple of weeks now, and most important now is to recover as much as I can, so maybe the extra day of rest I will get over Novak could be an advantage.
"I know physically where I am, and mentally I have a lot of confidence.
"I know my body is keeping it up.
"I still have another week where I have to give it all."
Djokovic has his own reasons to desire a successful defence of the Tour Finals title after losing his world number one spot to a resurgent Rafael Nadal.
Since winning the Australian Open in January, the 26-year-old has fallen just short at the Grand Slams, including final defeats at Wimbledon and the US Open to Andy Murray and Nadal respectively.
There is a slim possibility Djokovic could regain the number one ranking depending on results in London, but he refuses to focus on that.
"I know there is a small chance to finish number one in the world this year and it would be an incredible achievement," he said.
"But it does not depend on me only, it depends on Rafa as well.
"I don't think it's very possible, so I just try to focus on building something that can pay off in the next season."
Nadal, in Group A against David Ferrer, Tomas Berdych and first-time qualifier Stanislas Wawrinka, also has an immediate chance to erase a frustrating loss as he starts the Tour Finals against fellow Spaniard Ferrer, who defeated him in the Paris Masters semi-finals on Saturday.
Nadal has never won the Tour Finals, losing his only final against Federer in 2010, but he will be a strong contender this year after a superb run of late, especially with Wimbledon champion Murray absent due to a back injury.
"There is one tournament remaining so I will try to play with the best attitude I can," Nadal said.
"I know at the end of the season a lot of times my body and mind find it tougher.
"I arrived in Paris a bit more tired, but I'm going to try to play better in London."
Berdych and Wawrinka will open the tournament on Monday afternoon, with Del Potro, who fell victim to thieves as he left Paris on Saturday, playing Gasquet in the evening.