Roger Federer and Rafael Nadal said they welcome innovation in tennis but warned the game's administrators on Friday not to tamper too much with the current format.

A series of rule changes and innovations are being trialled at the Next Gen ATP Finals in Milan, featuring the world's top 21-and-under singles players.

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They are aimed at creating a high-tempo, TV-friendly product geared towards attracting new and younger fans.

Rule changes include a shorter format with first-to-four-games sets (tie-break at 3-3), a shorter warm-up, a 25-second shot clock used between points, no line judges, player coaching and no doubles lines on the court.

Federer, in London for the ATP Finals, starting on Sunday, said he had enjoyed watching the matches on TV but urged caution.

"We need to think, take seriously all these rule changes if ever you're going to do it because once you do it you don't want to bounce back and forth with changing something and then you don't like it later on," said the Swiss world number two.

"I don't see that much wrong with our Tour right now that it needs that much fixing, especially the shorter sets. I know it can be somewhat intriguing but at the same time the longer sets allow you to stretch a lead, it's more comfortable at times.

"You can try different things. You can work on stuff, whereas if every point counts so much... there's no room for anything any more. There are positives and negatives to it but I don't want to see anything change on the Tour that much to be honest."

Top-ranked Nadal, bidding to win the ATP Finals for the first time, said it was good to try new things.

"There are a couple of things that I like and a couple of things I don't like but nothing is perfect. We are in a sport where we have a big tradition -- not many changes have been made in all of its history."

The Spaniard used the example of net height, which has stayed the same despite taller players in the modern era.

But the 16-time Grand Slam champion, who looks likely to play in London despite an injury scare , said he was happy with how things are.

"If you asked me 'do you want changes?' I will say 'no'," he said. "I am number one in the world, I have achieved a lot of things but if the game needs something to be more attractive for the fans, that's the way that the game will move forward."

"It's not my job," he added. "The people who run the sport have to look around and take the right decisions to make our sport even more interesting than it is today. I'm happy with how it is but maybe in the future you need to do something."