Murray had been the clear favourite following Novak Djokovic’s early exit but the Scot was out of sorts as world number 50 Zverev sealed a shock 7-5 5-7 6-2 6-4 victory.
Asked if he felt flat, Murray said: “I don’t think I was flat. I feel like that’s something I get asked all the time when I lose matches. Were you flat? I don’t think I was today. I was getting myself pumped up. Sometimes at the end of the sets I was trying to get a little more energy, show more sort of positive body language.
“And I did that at the end of the match, at the end of the first and second sets. It just wasn’t to be today.”
Murray’s loss, coming three days after defeat , has thrown the draw wide open, with the likes of Federer, Raonic, Rafael Nadal and Stan Wawrinka primed to capitalise. Federer warned his rivals he feels fit and ready to go the distance after beating Kei Nishikori in five sets to reach the quarter-finals.
This was only Federer’s fourth official match since returning from six months out with a knee injury but the Swiss stayed tough to win 6-7 (4/7) 6-4 6-1 4-6 6-3 in three hours and 24 minutes. He now continue his pursuit of an 18th grand slam against Mischa Zverev tonight, the world number 50 who had earlier pulled off a shock win over top seed Andy Murray.
Federer’s previous five-set match came at Wimbledon last year when he was beaten by Canada’s Milos Raonic in the semi-finals, two days after winning a decider against Marin Cilic. The 35-year-old said he felt fresh against Nishikori and was relishing, the decider.
“I felt great in the fifth, I must say,” Federer said. “Great energy. Even deep into the fourth I thought, ‘Yeah, fifth, here we go, no problem for me. I’m feeling good about my chances’.
“I was playing positive tennis, I was playing offensive. My body was reacting.
“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 - understanding the comeback really.
“I think now 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, which I’m very happy about.”
Defending women’s singles champion Angelique Kerber bowed out in the fourth round after being blown away by American Coco Vandeweghe.
Vandeweghe was a worthy winner too on Rod Laver Arena, winning 6-2 6-3 in just 68 minutes to book a quarter-final showdown with Garbine Muguruza.
Ranked 35th in the world, the American lost in the first round here 12 months ago, just as Kerber was beginning her path to become champion.
The world number one, however, has endured a difficult start to 2017 and her fragile confidence was ruthlessly exposed here by a powerful display of baseline hitting. “It was really special,” Vandeweghe said. “Beating the world number one on any stage and any place is great... I’ll take this one.”
Kerber’s defeat continues her poor start to the season. “I made a lot of mistakes. I think that was not my game I play normally,” Kerber said.
“It was a tough match and of course I’m disappointed. But I was not feeling the ball at all tonight. I was not playing good from the first point. So yeah, it was not my day and not my match, for sure.”