'Tough to watch': Tennis world stunned by 'distressing' on-court meltdown

Alexander Zverev has left the tennis world stunned with more bizarre on-court theatrics at the ATP Cup.

Stefanos Tsitsipas crushed a furious Zverev but the Greek spearhead couldn't quite finish the job in the decisive doubles as Germany's tense ATP Cup win ensured Australia early passage to the quarter-finals.

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World No.6 Tsitsipas exploited Zverev's serving woes in Brisbane on Sunday night in a 6-1 6-4 dismantling of the seventh-ranked German, who threw in 10 doubles faults and made just 45 per cent of first serves.

Zverev’s bizarre on-court meltdown continued on Sunday after suffering similar issues against Alex de Minaur in the previous tie.

Zverev will hope his serve improves in their final clash with Canada after managing 14 double faults in a first-up loss to De Minaur on Friday.

Alexander Zverev was in full meltdown mode. Image: Channel Nine

On Sunday he raged at his father and coach Alexander Senior between games as his serve crumbled, while team captain Boris Becker awkwardly stood by.

Tennis fans and commentators were stunned by the extraordinary scenes.

Tsitsipas then paired with world No.487 Pervolarakis in a thrilling doubles clash with French Open champions Kevin Krawietz and Andreas Mies that was decided 3-6 6-3 17-15, the Greeks missing six match points in the tie-break.

The overall German victory kept their faint finals hopes alive and ensured Australia, who will meet Greece on Tuesday, topped their pool after back-to-back 3-0 wins against Canada and Germany.

Russia spank John Isner’s Americans

Meanwhile Russia took a giant step towards booking their last-eight berth by beating the US in their tie in Perth on Sunday night.

Karen Khachanov came from a set down to beat Taylor Fritz 3-6 7-5 6-1 before world No.5 Daniil Medvedev destroyed John Isner 6-3 6-1 in just 59 minutes.

Medvedev made just two unforced errors for the entire match, and he also served more aces than Isner - two stats he was left amazed by.

Alexander Zverev and Boris Becker. (Photo by PATRICK HAMILTON/AFP via Getty Images)

"It's the first time I've played him and to be honest I was very scared of his serve," Medvedev said.

"I knew I had to just wait for my chances. Maybe it wasn't John's best day of serving, but I'm happy to get through.

"That (to serve more aces) is a great achievement. If you would have asked me before the match, I wouldn't have thought it was possible."

Russia will book their spot in the quarter-finals with a win over Norway on Tuesday, but even a loss might be enough to sneak through.

The US are now out of contention after losing their opening two ties - the first a shock loss to lowly Norway.