LONDON (AP) – Roger Federer says in an interview with The Associated Press that he thinks impossible to compare the careers of top players such as him, Rafael Nadal and Novak Djokovic. Federer is preparing to retire at age 41. He has 20 Grand Slam titles, which was the record until it was surpassed this year by Nadal, who is now at 22, and Djokovic, at 21. Federer says the debate over which of the so-called Big Three is the greatest of all-time is a “fun debate” but also “silly.” Federer says he will keep tabs on Nadal and Djokovic as they pursue more major championships and that he hopes they will go far.