Federer won at Wimbledon last year aged 30 and the 17-time Grand Slam champion continues to be ranked among the top three players in the world.
He told BBC Sport: "Thirty-one years of age is quite old to be a world class tennis player.
"He clearly works extremely hard and never gets injured. It's the freshness. He just loves the game.
"He may be a third of a step slower than he was five, six, seven years ago but he's flourished with the rivalry of Nadal, Djokovic and Murray and that rivalry has reinvigorated him. It's thrilling to see him competing against the younger guys."
Federer goes up against Murray in the Australian Open semi-finals this morning for the right to play Djokovic in Sunday's final.