Pete Sampras has expressed his surprise at the continued success of Roger Federer, despite his age and family commitments.

Federer battled through to the third round of the Australian Open after beating Simone Bolelli, and Sampras, a 14-time Grand Slam winner, has hailed the motivation of the 33-year-old father of four.

He told reporters: "It's amazing that Roger is still going. He has done everything in the game and then some, and yet he still has the motivation to travel and to win.

"Am I surprised that Roger is still up there? Yes and no. I don't doubt Roger's ability. That's still there. He's been playing well. But it's the mental side. I'm just amazed that he still has the motivation, that he is still travelling and still winning events."

Federer is searching for his 18th Grand Slam in Melbourne.