Roger Federer has won the ATP World Tour Finals six times. (Clive Brunskill/Getty Images)

Roger Federer doesn't share Rafael Nadal's opinion that the surface for the ATP World Tour Finals should change every year.

In an interview with CNN, Federer said he believes the season-ending tournament should continue to be played on indoor hard courts. Since a two-year run on outdoor hard courts in Houston from 2003-04, the World Tour Finals have been played on indoor hard courts in Shanghai (where carpet was used one year) and now London. They've never been played on clay and were contested on grass only once, in Melbourne in 1974.

"I know there's an argument to have it on clay, on grass, to have it outdoors -- and I did play outdoors in Houston," Federer said. "I just feel indoors doesn't have enough play. The indoor season is small. I believe indoors deserves a huge event, which this one is."

The World Tour Finals completes a fall indoor stretch that featured the Paris Masters last week and ATP 500 events in Valencia, Spain, and Basel, Switzerland, two weeks ago. The ATP also had two 500 events on indoor hard courts in February.

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Federer has won the World Tour Finals six times. Nadal, who this week is making his sixth appearance, has yet to win the eight-player, round-robin event. Indoor hard courts are the weakest surface for the Spaniard, who said he's been "unlucky" in being unable to play the World Tour Finals on a different court. Nadal questioned the fairness of keeping the surface the same year after year.

"In the World Tour Finals, we qualify on all surfaces," Nadal said Monday. "It would be a little bit great -- I’m not talking about me because I know it won’t happen for my generation, so I’m talking for the next generations -- [to have] something more fair to the players and I think for the fans it would be something more interesting to change the surface every year.”

Tournament director Andre Silva told CNN that he's open to the "interesting concept" of changing surfaces but questioned the practicality.