LONDON -- Roger Federer has won one title in the past 14 months, at a low-key tournament this summer. The ATP Finals are his last chance for a big victory this season.
"I still have another week where I have to give it all. After, it will be long holidays," Federer said after losing to Novak Djokovic 4-6, 6-3, 6-2 on Saturday in the Paris Masters semifinals in their first meeting this year.
The pair was placed in the same group at the season-ending, eight-man finals, which start Monday.
Djokovic, who won his 16th straight match in Paris against Federer, is the defending champion in London, where he beat Federer in the final last year. Djokovic also won the title in 2008, while Federer has won it a record six times.
Federer said he will focus on his recovery ahead of Tuesday's match against Djokovic.
"I'm always excited about those challenges," said Federer, who will be competing for the 12th straight year in the ATP Finals. "I have had a good couple of weeks now, and the most important thing is to recover as much as I can, so maybe the extra day I will get over Novak now, who knows, could be an advantage. But I doubt it."
Federer, who won his last Grand Slam title at Wimbledon in 2012, has only two victories against top-10 opponents in 2013. He registered the first one in January against then-No. 8 Jo-Wilfried Tsonga in the Australian Open quarterfinals before a series of defeats against opponents outside the top 50. He confirmed his recent improvements with a win over del Potro in Paris this week.
After reaching the final in Basel for his first final in more than four months in October, Federer showed more signs of recovery in Paris, where the back problems that have been hampering him for most of the season did not reappear.
"I played a lot of tennis during the past 10 or 12 days. It gave me a lot of information," Federer said. "I know what I have to do now. I know what I did well, what I can improve. I know physically where I am, and mentally I have a lot of confidence. I know my body is keeping it up."
Del Potro said he was the victim of a theft at a train station in Paris on his way to London, having his suitcase stolen as he stopped to sign an autograph. The tall Argentine lost his passport and a personal item in the incident.
"I don't know who asked me for an autograph but I turned round and they took my suitcase," he said. "But I got here and it happens in every country. I have my rackets and my clothes but nothing else. I lost personal things, my rosary that the Pope blessed in Rome, which is important to me. But it's over and I'm safe."
Saturday's draw placed top-ranked Rafael Nadal in Group A alongside fellow Spaniard David Ferrer, Tomas Berdych and Stanislas Wawrinka. Nadal lost to Ferrer in the semifinals in Paris and will have a chance to avenge his loss when he plays him again in London on Tuesday.
The finals have a round-robin format, with the top two finishers reaching the semifinals.
Berdych and Wawrinka will open on Monday, followed by del Potro playing Gasquet.
Wimbledon champion Andy Murray pulled out after having back surgery.