LISTEN TO GRAEME AGARS TALK WITH D'ARCY AND GORAN ABOVE
World No. 4 Alex Zverev completely lost the plot in an extraordinary outburst following his early brain snaps against Milos Raonic.
The rising superstar dropped the first set to Raonic after a weird start to the match saw each player's serve broken within the first 10 minutes.
A particularly silky Raonic pulled away and capitalised on a number of errors from the 21-year-old and stole the set 6-1. Cleary filthy at his series of errors and failures to find his way into the match, Zverev finally snapped when he went down 5-1 in the second set.
The German star smashed his racquet into the ground nine times during a change of ends, an outburst that's already being labelled one of the ugliest in recent years.
"That's crazy. That's like twilight zone stuff," Tennis legend John McEnroe said.
The American champion said Zverev's stroppy demeanor on court would have been a sports psychologists dream.
"Right now he is not exuding positive energy. He's moping a little bit," McEnroe told Channel 9.
"And he's down a set, down a break. Things are not going well. He is also giving positive feedback to a player who doesn't need any more than the scoreboard is giving him. It's a different energy than a lot of other top players would give off. Certainly Rafa wouldn't be doing anything other than coming out of his skin trying to get back into it.
"Just generally speaking the body language, he's got the racquet sort of drooping towards the ground. He's not sticking his chest out at all. He gives himself a sarcastic thumbs up there. A sports psychologist would do a lot with this little clip of Zverev as far as things not to do in this environment."
Fellow Channel 9 commentator Todd Woodbridge said of Zverev's poor performance: "It's all just a little lazy".
Earlier, Naomi Osaka is through to the quarter-finals of the Australian Open for the first time after she had to come from a set down against Anastasija Sevastova.
Both players showed moments of tightening up in a tense final set, but it was Sevastova who let her mental demons overcome her.
The under dog, who is notorious for her stroppy, self-defeated body language, was back to her old tricks after she fell behind in the final set, triggering concern for her emotional state.
Some tennis commentators even said the 28-year-old was "falling apart" on Rod Laver Arena.
In the end she was unable to recover and fell 4-6 6-3 6-4 after taking the first set.
Sevastava found herself down 3-1 in the decisive set before she secure the break back in a marathon sixth service game where the scores were levelled 3-3 when the Latvian converted her sixth break point opportunity.
Sevastova cut a forlorn figure just minutes later when her next service game was broken and the No. 13 seed continued to show her immense frustrations at her own unforced errors at a crucial point in the match.
It was all Osaka needed to go on and serve pout the match un just under two hours.
Meanwhile, Roger Feder has sharply rebuked a comment from John McEnroe, who declared we were "watching the changing of the guard" as Greek youngster Stefanos Tsitsipas eliminated Federer from the Australian Open on Sunday night.
"Yeah, sure," Federer said at his press conference, when asked about McEnroe's assertion during an on-court interview with Tsitsipas.
"He's in front of the mic a lot. He's always going to say stuff. I love John (but) I've heard that story the last 10 years. From that standpoint, nothing new there."
While not ready to agree his time as a grand slam contender is up, Federer was happy to welcome Tsitsipas to the "next level".
"I think he's definitely done a really nice job the last year and a half," Federer said.
"I mean before that, too, obviously. But beating Novak (Djokovic) in Toronto, the likes of (Kevin) Anderson and (Sascha) Zverev, now me here. That's what you need to do to get to the next level. He's doing that. It's really nice for him.
"I see him definitely being high up in the game for a long time. That was a good night for him tonight."
Tsitsipas' 6-7 (11-13) 7-6 (7-3) 7-5 7-6 (6-5) victory on Rod Laver Arena sees him progress to a quarterfinal against Spain's Roberto Bautista Agut.
He was also reluctant to read too much into his victory.
"I try to take every match differently. For sure it's a good win against Roger. I mean, we all know who Roger Federer is, what he has done in tennis. But I still have to keep my focus, keep my concentration on further goals that I want to achieve. That's a very good beginning. I need to stay humble," Tsitsipas said.
"This win is a good milestone, let's say good first step, as I said, to something bigger. I do feel like my game is pretty good at the moment. I feel confident. That's very important. I'm really pumped and excited to be competing in the quarterfinals two days from now. I'm really waiting for that moment."
Federer's fourth-round loss follows a similarly shocking round of 16 defeat at last year's US Open to Australia's John Millman.
The last time he went back-to-back majors without making a quarterfinal was way back in 2013, when he lost in the second round of Wimbledon to Sergiy Stakhovsky and the fourth round of the US Open to Tommy Robredo.
In a somewhat surprising announcement, Federer plans to respond to his Melbourne Park failure by playing clay court tournaments for the first time in the past few years.
Despite appearing to benefit by a reduced schedule in recent years, Federer appears ready to return to Roland Garros, where he's been missing since the 2015 French Open.
"I'm at a moment where I think it would be nice to do it,'' said Federer, speaking in French. "I can say I have missed it, I did the right thing skipping it last year, the year before as well, and the year before that I was injured. So I felt I wanted to do it again."
Just one of his 20 Grand Slam titles has been on clay — in 2009 when he beat Robin Soderling in the final. He lost the three previous French Open finals to Rafael Nadal.
Federer said he wants to avoid having a long break in the middle of the year before the grass season.
"I feel that it is not really necessary,'' he said. ''So that's how this was decided.''
His fans will hope it's an attempt to help rebuild a world ranking that will fall from No 3 to at least No 6 following this tournament — rather than a sign 2019 is looming as a farewell tour.