LONDON—Rafael Nadal once again comes out in favor of switching surfaces at the ATP World Tour Finals. Nadal, who has won Grand Slam and Masters Series tournaments on every major surface, has said that the current choice, fast indoor hard courts, does not suit his game. Nadal has never won the ATP World Tour Finals.

“One of the big reasons is I have not always been a fantastic player on indoor,” said Nadal, who also added that year-end fatigue is also a factor. “Just a few times have I been able to play at a high level on indoor. The third thing is because I feel like I am unlucky. During these nine years the Masters Cup was on indoor, a surface that was not the best for me. So probably for me I was a little bit unlucky in this because in the past we had the Masters Cup on other surfaces. I understand, but I think this is unfair.”

Roger Federer, who has won the ATP World Tour Finals six times, disagreed, telling CNN it should stay indoors.

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

ATP World Tour Finals tournament director Andre Silva said the tour would need to consult the players first.

"If it's something the players want and can be done logistically, it's something we can explore," said Silva. "Even in the same place it would be pretty unique if you have all of a sudden one year on a hard court in London and the next year on a clay court. But it's hard for players to switch surfaces. I think guys who haven't played on clay for six months wouldn't be comfortable going back to a surface they haven't experienced in six months. I'm not opposed to it but I just think it becomes hard because of the logistics."