Not so this year at the Barclays ATP World Tour Finals, when the best eight men in the world — sans injured Andy Roddick— gather in London at the splashy 02 Arena.

For the first time since 2003, the year-end No. 1 ranking is up for grabs. However, it won't be easy to wrest that slot away from four-time winner Roger Federer, who arrives in the clear pole position.

Sunday's opening matches in the eight-day, round-robin event pit local favorite Andy Murray against U.S. Open champ Juan Martin del Potro and Federer vs. No. 8 Spaniard Fernando Verdasco.

With a 945-point lead on rival Rafael Nadal and a maximum of 1,500 ranking points to an undefeated winner, the reigning French Open and Wimbledon champ from Switzerland would have to stumble badly — as he did in an opening-round loss at the Paris Masters last week — to give No. 2 Nadal a chance to surpass him.

"It's pretty stacked in Federer's favor," Tennis Channel commentator Justin Gimelstob said.

Because of the quirky round-robin format, each win or loss has an impact on a player's final ranking points.

Suffice it to say, Nadal must win the title with a perfect 5-0 record, and Federer must go 0-3 in round-robin play or fail to advance past the semifinals after notching a 2-1 record in preliminary action.

If Federer, 28, succeeds in finishing the season No. 1, he will add two more notables to his already overstuffed résumé.

The 15-time major winner and father of new twin girls would become just the second player after Ivan Lendl in 1989 to reclaim the top slot since computer rankings began in 1973. Federer finished No. 1 from 2004-07 before Nadal broke his stranglehold in 2008.

Federer also could tie American Jimmy Connors with five year-end No. 1s, which would leave him one behind all-time leader Pete Sampras.

With a total purse of more than $5 million and up to $1.63 million for an undefeated champion, London provides plenty of monetary motivation. But who will emerge with the year's final spoils is anybody's guess, especially with many players tired, banged up, or psychologically winding down after a long season.

Aside from Federer's victories in 2003-04 and 2006-07, the year-end championships have often produced surprises, such as James Blake's run to the finals in 2006 and David Ferrer's runner-up finish in 2007.

Defending champ Novak Djokovic has had the recent hot hand after a season of Grand Slam disappointments. The No. 3 Serb captured three of four events he entered this fall, including last week's Paris Masters, but he also has logged a tour-high 96 matches. Fatigue could be an issue.

Argentina's del Potro, just 21, has struggled since knocking off Federer in the New York final. He's 2-2 since winning his first major.

Fourth-ranked Murray won a tour-leading sixth title at Valencia this month but has played sparingly this fall since re-aggravating his left wrist in Davis Cup in September. Murray can count on one thing: major hometown support.

Australian Open winner Nadal's aching knees seem to be holding up and he has played solid the last two months, advancing to at least the semifinals in three events. But the 23-year-old Mallorca native will have to take it up a notch to overtake Federer.

Others, like big-hitting Swede Robin Soderling, who replaced Roddick, and Shanghai Masters winner Nikolay Davydenko of Russia, are dangerous, if outside, title contenders.

One thing that has been a lock is buzz around the event. Sales at the 17,500-seat O2 arena have been robust, no doubt because of the wildly popular Murray, who hails from Scotland.

According to tournament chairman Phil Anderton, only 9,000 of the event's original 263,000 tickets remain.

"Murray is a big draw," Anderton said by phone Thursday from England. "For a lot of people here, tennis is Wimbledon," added Anderton, who is the ATP's chief of marketing. "To have the top eight players coming in November to cold, dark, dank London playing indoors — it has people intrigued."

This is the eighth time since 1990 that the top ranking has come down to the year's final tournament.

The USA's Roddick, who pulled out this week with an injured knee, was the last player to enter the year-end championships with his top ranking on the line. He managed to lock it up by reaching the semifinals at Houston in 2003, the same year he won his sole major at the U.S. Open.

"This is exactly how we want it," Gimelstob said. "It's coming down to the wire."

Enlarge By Jacky Naegelen, Reuters Roger Federer of Switzerland enters the ATP World Tour Finals, which begin Sunday in London, with a firm hold on the year-end No. 1 ranking.