If they do happen to meet, it will be just the third time they will have played this season, which started with their tense Australian Open final. After playing 15 matches in the previous three years, Federer and Nadal encountered each other fewer times in 2009 than they have since their rivalry really started to heat up.
Andy Roddick's withdrawal will make this the first ATP year-end tournament without an American participant since 1986.
Roger Federer, 28, Switzerland
What's at stake: This peerless champion will be faced with the same questions in every major tournament he plays for the rest of his career: Why are you still here? What are you playing for? Where do you find motivation? Perhaps it's fortunate for him that the year-end No. 1 is on the line in this event, providing concrete inspiration. Even if Federer is understandably running out of gas as this dramatic season staggers to a close, he might be able to convert pride into fuel after two recent, surprising losses: one in his hometown event in Basel, Switzerland, and one in the Masters Series tournament in Paris. He is 27-5 at this event and a four-time winner (2003, 2004, 2006, 2007).
Rafael Nadal, 23, Spain
What's at stake: This really isn't the year-ender for Nadal, who could be forgiven for looking ahead to Spain's Davis Cup final against the Czech Republic in Barcelona a week later -- or the fact that he'll have barely a month to recover before beginning the run-up to a defense of his Australian Open title. The slightest twinge during round-robin play in London might prompt Nadal to save himself for another day. This is the time of year when Nadal tends to struggle physically, but he's 20-6 since coming back from an injury hiatus last summer, so maybe that rest is paying off now. Five of those six losses were against men he could face in London. Nadal has reached the year-end semifinals twice, losing to Federer both times.
Novak Djokovic, 22, Serbia
What's at stake: Lost in the focus on Federer and Nadal's tug-of-war for the No. 1 ranking was the quieter rearguard action going on all season between Djokovic and Andy Murray for the No. 3 and No. 4 slots. It looks as though Djokovic -- the defending champion in this event and the hottest player on tour right now, with three titles in his last four tournaments -- will hang on to that bronze-medal position for the third year in a row. His recent streak has helped redeem a year in which Djokovic seemed to run in place, with less-than-satisfying results in Grand Slam events. At the risk of stating the obvious, he's the player to beat in London.
Andy Murray, 22, Great Britain
What's at stake: Will the home crowd help or stifle Murray? The pride of Great Britain won his first title on home turf last summer at the Queens Club, and although his popularity isn't apt to wane either way, winning in London would represent another step toward the top of the game. It's also worth noting that Murray is 13-1 in indoor play this season (and 72-17 in his career), with 2009 titles in Valencia and Rotterdam. Murray advanced to the semifinals in his first year-end championships appearance last year.
Juan Martin del Potro, 21, Argentina
What's at stake: It's hard to expect much of DelPo, whose train has come to a screeching stop rather than building up steam after his gutsy win over Federer in the U.S. Open final. The big Argentine has been more vulnerable than powerful since then, playing only five matches and twice retiring because of injuries. His name barely would have been mentioned the past two months had he not beaten Marat Safin, who had one foot off the court already, in Paris. We'll see if del Potro is up for any kind of effort here. He did not advance out of round-robin play last year in his debut at the year-ender.
Nikolay Davydenko, 28, Russia
What's at stake: He just keeps going and going, and will finish in the top five for the fifth straight year. In fact, Davydenko has remained in the top 10 since mid-2005 except for a 10-week spell this spring and summer. It has been a meandering year for the veteran, who won three lower-profile tournaments but didn't fare so well on the big stage (although he did beat four quality opponents to win in Kuala Lumpur). He hasn't shown much since a crisp straight-sets defeat of Nadal in Shanghai last month. Davydenko was the runner-up to Djokovic at the ATP championships last year.
Fernando Verdasco, 26, Spain
What's at stake: Verdasco moved from the fringes of the ATP elite into the inner sanctum based on a strong start and consistent play, although his winning pace has slowed since he took his only 2009 title in New Haven and reached the U.S. Open quarterfinals. He has the same distraction looming as Nadal (Davis Cup) but less wear and tear on his body. This is Verdasco's first trip to the finals; he is a combined 1-10 against Federer and Murray, who are in his round-robin group, and has never played del Potro.
Robin Soderling, 25, Sweden
What's at stake: It seems fitting that Soderling will play in this tournament for the first time in 2009. He became one of the year's big newsmakers by ending Nadal's streak at Roland Garros. He must have angered the tennis gods somewhere along the way, however. Soderling drew the Federer short straw four times this year, three of them in Slams. Three of Soderling's four career titles have come indoors, all on carpet. He has a career 1-8 record against round-robin opponents Nadal and Djokovic, but could be a spoiler if he bears down.
Bonnie D. Ford covers tennis and Olympic sports for ESPN.com. She can be reached at email@example.com.