When Roger Federer looks around at his opponents for the ATP World Tour Finals title, there are seven very familiar faces looking back.

A bit too familiar perhaps, for someone who is running out of new challenges in a record-breaking career.

With the season-ending tournament starting today with only one debutante in the field, Federer is among those wondering when the next great tennis talent will come along to shake up the world rankings.

So far, he sees no credible candidates.

"Promising newcomers? Not a whole lot, really," Federer said, summing up the lack of young talent threatening to break into the ATP elite. "I was asking myself the question too, why don't we have any teenagers in the top 100?"

That used to be quite commonplace in tennis, even in the top 10. Now though, Andy Murray and Novak Djokovic are the two youngest players to qualify for the London tournament — which features the top eight players on the circuit — at age 23.

Tomas Berdych, this year's Wimbledon runner-up, is making his debut at 25, while Federer is the oldest at 29 in his ninth straight appearance. Andy Roddick and David Ferrer are both 28, while Robin Soderling is 26 and top-ranked Rafael Nadal is 24 — and qualified for the sixth straight year.

"It's quite surprising for me, because when I was coming up there was (Lleyton) Hewitt and (Marat) Safin and Roddick and (Juan Carlos) Ferrero and Tommy Haas, they were all excellent players and in the top 100 as teenagers," Federer said. "It was normal, and on the women's side it was even more extreme. ... But maybe the game has become more physical and more mental and that's why maybe players today need a bit more time to break through."

Indeed, many players seem to reach their potential in their mid-20s these days. Berdych had his breakthrough when he stunned Federer in the Wimbledon quarterfinals this year before losing to Nadal in his first Grand Slam final.

Soderling made a similar splash by beating Nadal at the 2009 French Open, before going down to Federer in the final. He had the opposite experience this year, beating Federer in the quarterfinals before losing to Nadal in the title match.

Roddick, for one, isn't surprised by the slower developments.

"No, if you look at the way the game has changed, and how physically demanding it is now," the American said. "I weigh 20-25 pounds more than I did when I started out. But the game wasn't as physical, the conditions were a little faster.

"I think you had to rely a little less on your body when I was coming up, frankly. So I think it's a tough ask of a 17, 18-year-old to do it for 40 weeks a year. I think that's almost the main example that you can point to as far as what's changed."

And why the field at the ATP finals hasn't changed much.

Six of this year's players qualified last year as well, although Roddick had to sit out with an injury.

Last year's champion Nikolay Davydenko missed out this year after struggling with injuries and falling to No. 22 in the rankings. The 2009 runner-up Juan Martin Del Potro has missed most of the year after wrist surgery and is also absent.

The tournament begins with a round-robin group stage, meaning every player is guaranteed three matches. Murray plays Soderling in the first match today, before Federer takes on Ferrer. The other group starts play tomorrow.

Federer is looking for his fifth title in the event, and is hoping to end the year on a high.