LONDON: 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. 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.