World number one Novak Djokovic has labelled Roger Federer as the "ultimate challenge" ahead of their semi-final showdown at Wimbledon.

The defending champion beat Florian Mayer in straight sets to advance from the last eight, and will now meet the Swiss player after his 6-1 6-2 6-2 victory over Mikhail Youzhny.

Federer was in formidable form in the last eight, and Djokovic is preparing himself for a massive test.

"He's rated probably as the best player in history. He has won everything that a tennis player can win and he's coming back for more," said Djokovic.

"He has won the most grand slams in history so it is the ultimate challenge for me. Roger has been on the top of the men's game for so long.

"He has won six titles here and he definitely wants to prove to himself and to everybody else that he can win it once again."

Andy Murray will take on Jo-Wilfried Tsonga in the other semi-final.