World number six Alexander Zverev was pushed to the limit at the Australian Open on Thursday, needing five gruelling sets to beat Slovak qualifier Lukas Klein. 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 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.