When the Australian Open began here a week ago, there will not have been too many people who believed Roger Federer would be in contention for a place in the semi-finals.

But that’s exactly where we are at and the way the 35-year-old celebrated at the end of his five-set win over fifth seed Kei Nishikori here last night, he feels he can go even further.

Federer’s 6-7, 6-4, 6-1, 4-6 6-3 win put him into the last eight yet again and, considering his next opponent tomorrow is Mischa Zverev – and not Andy Murray – the Swiss will be feeling confident that he can go at least one round further.

“I was playing way better than the first couple of rounds where I put in a lot of mental energy to stay with my opponents, figure them out, how were they going to play, how were the conditions going to be, understand the comeback really,” said Federer, who, lest we forget, is playing his first competitive event in seven months.

“I think now that I'm in the tournament I was able to focus point per point on my opponent, on the tactics. That didn't drain me much.

“I felt great in the fifth set, which I'm very happy about. Started to play the good sets I was talking about that I knew I could. The question was could I hang with Kei till the very end. I was able to do that, so I'm super happy.”

World No. 1 Murray should have been his quarter-final opponent but he was taken out by German serve and volleyer Zverev, who was ranked outside the top 1000 at the start of 2015 because of two years out following wrist surgery.

It was as big a shock as the loss of Djokovic, beaten by Denis Istomin, ranked 117 and Federer was as surprised as anyone.

“That Novak and Andy are not there, that is a big surprise,” he said. “I never thought that Mischa Zverev and Denis Istomin would beat those two big guys.

“I guess it's good for tennis that a lot of guys believe stronger now that the top guys are beatable, are vulnerable, especially on a faster court. It happened completely in different circumstances. But two huge surprises. No doubt about that.”

Federer knows Zverev well and once beat him 6-0, 6-0 but said he would need to be wary of the German’s confidence.

“I've played him a couple times before, once on grass, once on clay, never on hard courts,” he said.

“Obviously he's on a high right now. He's feeling great. Probably feels the best he's ever felt on a tennis court. That's how I would feel after the win today against Murray.It's going to be tough and different and tricky. That's my mindset.”

Tomorrow’s other quarter-final will be between fourth seed Stan Wawrinka, who beat Andreas Seppi of Italy in three tiebreaks and No 12, who ended the run of Britain’s Dan Evans.

Women’s No 1 and the defending champion Angelique Kerber was blown away by Coco Vandeweghe, a big-hitting American who blasted 30 winners, knocking the German off the court in a barrage of outrageous shot-making.

The defeat means Serena Williams, who was due to play her fourth-round match last night, would overtake Kerber as world No.1 if she were to win the title.

Vandeweghe will now play seventh seed Garbine Muguruza in the last eight tomorrow.

“It’s really special, to play the No 1 in the world and I believe it’s my first No. 1 win so I’ll take that,” Vandeweghe said.