While fans and pundits continue to debate who should be considered the greatest men’s tennis player of all time, 23-times Grand Slam winner Serena Williams says Roger Federer gets her vote. Rafael Nadal picked up a record-extending 13th French Open title in October to join Federer at the top of the list of men’s Grand Slam winners with 20 titles, while world No. 1 Novak Djokovic, the youngest of the ‘Big Three’ at 33, is on 18. The 39-year-old Federer is this week playing only his second tournament since returning to the Tour in March after spending more than a year on the sidelines due to two knee operations. “I think two words sum it up: Roger Federer,” Williams, who turns 40 in September, told reporters at the Emilia-Romagna Open in Parma on Monday. “He’s just a synopsis of greatness and class and amazing and really changed the game. You …