Roger Federer and Novak Djokovic set for 50th meeting in Australian Open semi-final

Six-time champion came through a rollercoaster match and will meet either Djokovic or Raonic

Novak Djokovic was too good for Milos Raonic in their Australian Open quarter-final match. AFP

Roger Federer said he "believes in miracles" after saving seven match points in his Australian Open quarter-final win over Tennys Sandgren on Tuesday, and the Swiss third seed may need a few more when he faces defending champion Novak Djokovic in the semi-finals.

The six-time champion, struggling physically, saved three match points at 4-5 in the fourth set and four more in the tie-breaker to win 6-3, 2-6, 2-6, 7-6, 6-3. At 38-years-old, Federer is the oldest man to reach the Melbourne semis since Ken Rosewall 43 years ago.

"I believe in miracles," Federer said, revealing that he thought his groin strain had ended his chances. "There could be rain, there could be all kinds of stuff."

The Swiss great revealed at one point he thought "just let him finish me off in style" as he struggled through the pain. "I'm incredibly lucky tonight, today - I don't even know what time it is."

In comparison, Djokovic was ruthless in victory and, despite trouble with his contact lenses, tamed the big-serving Milos Raonic 6-4, 6-3, 7-6. as he targets an eighth Melbourne Park crown and 17th Grand Slam title.

The pair are all-conquering in Melbourne, sharing 12 of the last 14 titles between them. They have played each other 49 times, with the Serb in front 26-23.

"I have tremendous respect for Roger," said Djokovic, who became emotional in his on-court interview when asked about the tragic death in a helicopter crash of Kobe Bryant, a personal friend.

"The match-ups I have had against Roger and Rafa [Nadal] have made me the player I am today. May the best player win."

While Federer - into a record 15th Australian Open semi - has laboured through 14 sets in his last three matches, Djokovic has had a much easier ride, dropping just one set all tournament. But world No 3 Federer insisted he felt fine.

"It really depends sometimes how you're feeling inside, how much it takes away from you. But I must say I feel pretty good right now," said the 20-time Grand Slam champion.

His comeback against Sandgren was the stuff of legend, with the victory his 102nd at Melbourne Park, surpassing his tally at Wimbledon to make the Australian Open his most successful Grand Slam in terms of matches won.

After comfortably taking the first set, he felt the groin strain midway through the second. He wasn't moving well and making too many errors. He took a medical timeout but lost the set and the next one.

But he seemed to get a second wind and saved four match points at 5-4 down in the fourth set and another three before winning the tie-breaker and romping to victory as the adoring crowd went into a frenzy.

The fit-again Raonic reached the Melbourne semi-finals in 2016, when he lost to Andy Murray, and the quarters on three other occasions, including last year.

He had failed to beat Djokovic in nine previous attempts, but possesses a monster serve and was unbeaten in 59 straight service games heading into their clash.

That all changed when he faced his nemesis, perhaps the best returner in tennis.

"I felt great on court, I was very focused," said Djokovic.

He threatened straight away, earning break points on the Raonic serve several times before finally converting to take the set and snap the Canadian's run.

After the 63-game streak on his serve, his next amounted to just one game, broken again in the fourth game of the second set as Djokovic took a stranglehold.

The relentless Serb, who has won 11 straight matches this season, went toe-to-toe with the Canadian in the third set when at 4-4 he called for new contact lenses, which turned into a medical timeout. Raonic wasn't impressed that he didn't wait until the changeover.

Djokovic appeared to struggle with his vision on his return, but it quickly came back into focus as he raced through the tiebreaker to book his ticket to the semi-final.