Novak Djokovic has "lost his edge" and is "a shadow of what he was at his peak", says two-time 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.