(CNN) -- Tennis' year-end championships are supposed to showcase the top eight players in the world but Monday's opening matches in London took a backseat to another butting of heads between the two biggest names in the sport.
The difference of opinion came as Stanislas Wawrinka beat Tomas Berdych 6-3 6-7 6-3 in the afternoon and Juan Martin del Potro defeated Richard Gasquet 6-7 6-3 7-5 in the nightcap.
Having disagreed about a potential two-year ranking system and a previous selection of the ATP's executive chairman, the latest dispute between Roger Federer and Rafael Nadal revolves around the surface for the World Tour Finals.
Switzerland's Federer says the tournament should remain on an indoor hard court but Spaniard Nadal wants it played outdoors and on surfaces including his preferred clay.
Federer and his attacking game flourish under a roof -- he was unbeaten on indoor hard courts for two years from late 2010 -- and the 17-time grand slam winner boasts a career winning ratio of over 80% when playing inside.
Nadal's indoor strike rate, meanwhile, stands at about 66% -- and the world No. 1 has even less success on carpet. Clay is undeniably his most successful surface, with eight of his 13 grand slam triumphs coming at the French Open.
Nadal has never won the ATP World Tour Finals while Federer has on six occasions, with four of those coming indoors -- Houston in 2003 and 2004 proving the exception.
"I believe it should be indoor and hard," Federer, president of the ATP Players Council, told CNN on Sunday. "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. The indoor season is small. I believe indoors deserves a huge event, which this one is."
Playing the tournament in such fashion is a natural extension of the European "indoor swing" that begins in the middle of October.
Prior to London, Shanghai staged the year-end showpiece on an indoor carpet and then on an indoor hard court from 2005 to 2008.
The last time it was contested on a surface other than hard or carpet was way back in 1974, on grass in Melbourne, and Nadal insists it is time for a change.
The ATP, he says, should be "fairer" with the players.
"I know it's not going to happen for my generation," Nadal, who stepped down as vice-president of the players council last year, told reporters on Monday.
"I'm talking for the future generations, something that will be a little bit fairer for the players. And I think for the fans it will be interesting to change the surface every year to the surfaces we qualified on for the (World Tour Finals).
"A good player has a chance to qualify for the (World Tour Finals) four or five times in his career, so if during this four years you have a different surface every year, a minimum one time he will have the chance to play on his favorite surface."
In an interview with CNN last week, Andre Silva, tournament director of the World Tour Finals and the ATP's chief player officer, said changing surfaces would make for an "interesting concept" but presented some problems.
"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."
When asked about the issue by reporters, Novak Djokovic -- the third most successful active player in the men's game -- didn't "know what to say rightly."
The Serbian, however, said the championships shouldn't be held in the same city for more than three years.
London's stint will hit seven seasons come the end of its term in 2015 and Silva said there was "definitely" a possibility of a further extension. Federer told CNN he wasn't opposed to the idea.
On Tuesday, the Swiss enters the action when he meets Djokovic in the evening session with Nadal facing compatriot David Ferrer earlier in the day.