“There was intensity, of course,” he said. “We played four and-a-half hours. It’s just that, you know, it’s one of these days when you don’t feel that great on the court, don’t have much rhythm, and the player you’re playing against is feeling the ball very well.

“I started the season very well. Again, it’s a tennis match. On a given day, you can lose. I mean, nothing is impossible.

“What can I do? I did try my best till the last shot, but it didn’t work.”

After winning his maiden French Open to complete a sweep of grand slam titles, Djokovic’s form fell away in the second half of last season, prompting queries about his motivation leading into the new season.

The manner of Djokovic’s exit in Melbourne, where he has reigned supreme for most of the last decade, raised fresh doubts about the Serb’s mindset.

“At the moment I just want to go home, spend time with my family, and that’s all,” he said, adding the defeat was hurting him as much as any in the past.

Istomin, 117 in the world, is coached by his mother and wears glasses for ­eyesight problems. But after his sensational dethroning of Djokovic, the unlikely hero could ­finally become a star, at least in his homeland of Uzbekistan.

“It means so much for me to beat the world number two,” said the 30-year-old journeyman, who only got into the tournament main draw as the winner of the Asia wild-card play-off.

Istomin, who will play Spanish 30th seed Pablo Carreno Busta in the third round, joked that he saves money by having his ­mother, Klaudiya Istominaas, as his travelling coach.

“When your family is part of your team, it’s great. I was lucky that my mother is coaching me,” he said. “The other good thing is that I don’t need to pay the coach extra, you know.”

Williams said being herself was the key to sweeping past Safarova and into the third round .

“I knew that I wanted to jump out in the lead. I knew I wanted to just be Serena. That’s what I’m good at doing, is being Serena.”

Radwanska became the highest seed to fall in the women’s draw. The former Wimbledon ­finalist reached the semi-finals at Melbourne Park last year and in 2014, but managed only eight winners to her opponent’s 33 and slumped to her earliest exit at the year’s first grand slam since her first-round elimination in 2009.

Hours after Djokovic crashed out, Rafael Nadal raised hopes in a late night match of filling the power vacuum after demolishing Marcos Baghdatis 6-3, 6-1, 6-3 to reach the third round.

Djokovic loomed as a potential semi-final opponent for Nadal but the Serb’s five-set upset by Istomin shook up the draw and left the Spaniard as the sole grand slam champion in the bottom half.

Nadal has had two lean years at the majors, but on the same Rod Laver Arena where Djokovic fell to Istomin, the 14-times major champion showed enough of the old passion and firepower to suggest he may yet go deep in the second week at Melbourne Park.

Additional reporting by Agence France-Presse