Novak Djokovic has "lost his edge" and is "a shadow of what he was at his peak", says 1987 Wimbledon champion Pat Cash.
The defending champion lost in five sets in round two of the Australian Open to world number 117 Denis Istomin.
The Serb, 29, won four Grand Slams in a row between 2015 and 2016 but has made only one of the past three finals.
"If we were doubting it before, we confirmed he's not the same player he was six months ago," said Cash.
Speaking to BBC Sport, the Australian added: "[Thursday's defeat] just shows that Novak has absolutely lost his edge, there's no doubt about that."
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Djokovic - who has slipped to number two in the world with Andy Murray taking the top spot in the rankings - completed a career Grand Slam in June 2016 when he beat Murray in the French Open final, but has struggled for form since then.
The loss to Istomin was Djokovic's earliest exit from the Australian Open since 2006, when he was beaten in the first round by American Paul Goldstein.
"I'd love to see him competing, to be a true number two fighting for that number one spot, but at this rate I don't think we will see that," Cash said.
"It's clear that it's a mental thing. He's done everything he needed to do and wanted to do in his career."
Djokovic denies lack of intensity
Six-time Australian Open champion Djokovic double-faulted nine times in an error strewn performance against Istomin.
He denied he had lost intensity since completing the career slam, adding that he "forgot about it" when he began a new season.
"It's not a time now to go so deep into it. I didn't reflect on that at all at this stage," he said.
"It's just that, 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."
Since winning the French Open in June, Djokovic has been knocked out in the first round of the Rio Olympics and lost both his US Open title and his status as world number one.
"It's not like this technique has fallen off. There's obviously a big confidence thing, but now he makes mistakes and it is a mental thing," Cash added.
"Maybe it's his time to say I'm not quite the same player that I was but I can still perform well."