Djokovic and Federer's 50th in Australian Open semi-finals

32-year-old Serb leads the Swiss superstar 26-23 in their head-to-heads

MELBOURNE: Roger Federer faces great rival Novak Djokovic for a 50th time in the Australian Open semi-finals on Thursday.

Swiss maestro Federer finds himself in the unfamiliar position of underdog when he takes on the reigning champion in Melbourne, where temperatures are set to soar to 37 degrees Celsius (99 Fahrenheit).

Nadal aims to keep heat on Federer Slams record

The 32-year-old Djokovic leads Federer 26-23 in their head-to-heads, and while the Serbian has moved serenely into the last four, Federer made it by the skin of his teeth.

The 38-year-old was two points from defeat against John Millman in the third round and, incredibly, saved seven match points in his quarter-final against Tennys Sandgren.

'Hopefully not this year' - Big Three aim to keep young challengers at bay

But Djokovic says that Federer's heroics against the 100th-ranked American are exactly why the 20-time Grand Slam winner is to be feared.

"What he did was amazing, he showed me he's one of the best players of all time," said Djokovic, who is targeting a record eighth title in Melbourne.

"I mean, he never gives up. When it matters the most, he's focused and he plays his best tennis.

"He's a great fighter."

The other semi-final, between Alexander Zverev and Dominic Thiem, is on Friday.