A smouldering Alexander Zverev's serving woes have worsened in an angry straight-sets loss to Stefanos Tsitsipas in the ATP Cup.
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Greek world No.6 Tsitsipas beat Germany's world No.7 6-1 6-4 in Brisbane on Sunday night to keep his country afloat in their tie, and the tournament.
Zverev cut an exasperated figure as he traded words with his team between games, before finishing with 10 double faults and a first serve success rate of 45 per cent at Pat Rafter Arena.
It followed 14 double faults in a three-set loss to Australia's Alex de Minaur on Friday night.
Zverev smashed his racquet and steamed after losing from one set and a break up against the Australian.
But he was never in that position in this high-profile clash - the first top-10 meeting in ATP Cup history - with the Greek dominating the first set and coming from an early break down in the second.
The win was Tsitsipas's first in the tournament and ensured their encounter progressed to a decisive doubles tie, after Jan-Lennard Struff thrashed Michail Pervolarakis 6-4 6-1.
The two best-performed second placed teams from the six groups spread across Sydney, Brisbane and Perth will join the six group winners in the quarter-finals in Sydney.
Greece were beaten 3-0 by Canada on Friday while Germany were swept by Australia, meaning the loser of Sunday's clash will drop to 0-2 and effectively out of quarter-finals contention.
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.
"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.
Australian Associated Press