The world No. 2 got off to a winning start against Jack Sock to kick-start the event at the O2 and was quizzed on previous comments made by the Spaniard.
Nadal moaned in the past that the tournament has never taken place on clay, and branded it as ‘not fair’.
‘I believe that it’s not fair that a player like me really never played on a surface that was a little bit more favourable. I always played on the worst surface possible for me,’ the 31-year-old said.
‘If it’s indoor at the 02, then great, but you can build a different type of surface there. It is a fantastic place to organise a World Tour Finals.
‘The atmosphere is just amazing and I am very happy it is there. I enjoy it every time I have had the chance to be there, but we can play on clay there too.’
But Federer defiantly hit back at those comments and believes there is a big enough clay-court season already.
‘Fair’? I’m not sure if it’s the right word,’ the Swiss said.
‘But I think it’s right and fair that it’s indoors, as well. There is no Masters 1000s on grass. There is one Masters 1000 indoors: Paris. So I feel like indoors also deserves its place, you know.
‘Could it be switched up to clay once in a while? Yeah, maybe. Could we have more 1000s on grass? Yeah, we could have that, too.
‘Could we have less on clay, more on hard courts? Yes, it’s all debatable.
‘I think it’s not the time of the year for clay, so there you have it. You can do indoor clay, I guess, but that’s a bit silly.
‘But I get his point, and it’s a fair point.’