Originally Posted by Protoss
I'd say 31 is old in tennis though.
Plus she's played 662 singles matches in her career. A lot of players start to decline after around 600 matches played.
Well, the average age of players in the top 100 is going up (or so I hear). And Serena played her 662 singles matches spread out over a very long time, giving her body plenty of time to recover.