Zverev taken to brink by Klein in five-set Australian Open thriller
Facing a second consecutive second-round exit in Melbourne, the German came back to win 7-5, 3-6, 4-6, 7-6 (7-5), 7-6 (10-7) after a gripping contest lasting four-and-a-half hours on Margaret Court Arena.
But it was a major wake-up call for the sixth seed against a player ranked 163, in only his second Grand Slam and who currently has no coach.
His reward is a third-round clash with unseeded American Alex Michelsen.
"I would much rather one-and-a-half hours, but what can you do? He played incredible," said Zverev.
"He was hitting every single ball as hard as he could from both sides and I didn't really know what to do.
"To be honest, he probably deserved to win the match more than me today, but that's tennis sometimes. Now I'm into the third round and it's about recovering."
Zverev, who is facing a trial in May for allegedly assaulting his former girlfriend in 2020, an accusation he denies, had also struggled in his first round match, dropping a set.
He started well against Klein, securing the only break in the first set to take charge. But it was short-lived.
Klein saved two break points on his opening serve in the second set then pounced with a baseline winner in the sixth game to break Zverev and level it at one set each.
The 25-year-old Slovak carried the momentum into the third set, immediately breaking with a net volley and never relinquishing the lead.
They went toe-to-toe in the fourth set, taking it to a nail-biting tie-break, where Klein's netted forehand on serve at 5-4 made the difference.
With the match on the line, Zverev saved a break point on serve early in the fifth set and responded by breaking Klein in the next game for 2-1.
But the gutsy Slovak would not go away, breaking back to level at 3-3 as the crowd roared them on before a tense tie-break finale.
