Novak Djokovic celebrates after beating Roger Federer during their semi-final match of the Australian Open in Melbourne on Thursday. AFP

Novak Djokovic beat an injured Roger Federer in straight sets to reach his eighth Australian Open final as unheralded American Sofia Kenin stunned top seed Ashleigh Barty to end home hopes on Thursday.

Djokovic twice fought back from a break down in the first set and after dominating the tie-breaker he never looked back, winning 7-6 (7/1), 6-4, 6-3 as Federer, who needed a medical time-out, faded.

"Respect to Roger for coming out tonight," said the Serb, who is closing on his 17th Major title.

"He was obviously hurt. He wasn't even close to his best in terms of movement."

Novak Djokovic pats Switzerland's Roger Federer after their match. Reuter

Djokovic, who has never lost a Melbourne semi-final or final, will now play the winner of Friday's clash between Dominic Thiem − who shocked world number one Rafael Nadal on Wednesday − and Alexander Zverev.

In the women's competition, Kenin, 21, upset Australia's Barty and Garbine Muguruza downed Wimbledon champion Simona Halep to join the American in Saturday's final.

Federer was hampered by a groin injury in his five-set quarter-final with Tennys Sandgren, and rumours swirled that he might pull out of the semi-final.

The 38-year-old Swiss, a six-time champion at Melbourne Park, was cleared by a scan but he took the court rating his chances at only "three percent" against Djokovic.

"I was playing with nothing to lose," Federer said, adding: "Today was horrible, to go through what I did.

"Nice entrance, nice send-off, and in between is one to forget because you know you have a three per cent chance to win."

Earlier, Barty had sent Australian expectations soaring of a first home-grown winner since 1978, but aggressive 14th seed Kenin saved two set points in each set to win 7-6 (8/6), 7-5 and battle into Saturday's decider.

Moscow-born Kenin looked stunned at reaching her first Grand Slam final, at a tournament where she has often gone unnoticed while fellow Americans Serena Williams and 15-year-old Coco Gauff hogged the headlines.

"I'm just speechless, I honestly can't believe this. I've dreamed about this moment since I was five years old... I've worked so hard to get here," said Kenin, who was a baby when her family moved to America with just a few hundred dollars to their name.

"She came up with some really great shots. I knew she wasn't going to give it up to me − I really had to fight out here. There's a reason she's number one."

Agence France-Presse