Roger Federer blasted critics who say he's too old and insisted he was confident of beating Novak Djokovic again at a Grand Slam after his four-set defeat in the Australian Open semi-finals on Thursday. 

The 34-year-old Swiss great was flattened by Djokovic in the opening two sets before he finally went down 6-1, 6-2, 3-6, 6-3, in his eighth loss to the Serb in their last 10 Grand Slam meetings.

Federer hasn't bested the runaway world number at a major since the Wimbledon semi-finals in 2012, when he last won a Grand Slam title. But when asked if he could beat Djokovic again on the big stage, he bristled. 

"Best-of-three, best-of-five, I can run for four or five hours. It's not a problem. I prove it in practice again in the off-season no sweat. So from that standpoint I'm not worried going into long rallies," Federer said.

"I know you guys (media) make it a different case. I get that, because you think I'm old and all that. But it's no problem for me. But it doesn't scare me when I go into a big match against any player who's in their prime right now."