Volkswacht Bodensee - Tsitsipas upset as Kazakhstan beat Greece to make United Cup knockouts

Greek star Tsitsipas is coming off a mediocre 2024 in which his ranking dropped to its current 11 from a career-high three.

He was hoping for a confidence-boosting fresh start at the mixed-team tournament in Perth but was stunned 6-4, 7-6 (7/0) by 78th-ranked Alexander Shevchenko.

World number six and 2022 Wimbledon champion Rybakina then survived a late wobble before burying Greek hopes with a 6-4, 6-3 defeat of Maria Sakkari.

In Sydney, Katie Boulter led Britain to victory over Argentina with all three teams in the group, which also includes hosts Australia, still in contention for the knockout rounds.

"I'm really happy for the team," said Rybakina, now working with Novak Djokovic's former coach Goran Ivanisevic. "We all tried hard.

"It was a tough match for me today and I'm so happy to be in the quarter-finals."

Rybakina took the first set and led 5-0 on the second, but the former world number three Sakkari won three games in a row to threaten a comeback.

Rybakina came through on her third match point as Sakkari mis-hit an overhead after 85 minutes.

"At 5-0 you have nothing to lose, she played more aggressively and hit some good shots," said Rybakina.

"I just focused one by one, I got a bit lucky on match point but it was a positive match for me."

Shevchenko called his defeat of a sluggish Tsitsipas "one of the best" of his career.

"He was not at his best today. But I played really well and found the moment to win that match," he said.

"It will definitely go in my record as one of my best wins."

Defending champions Germany, led by Alexander Zverev, face China in the Perth evening session.

in Sydney, world number 24 Boulter, coming off her best-ever season, took 86 minutes to take down 100th-ranked Nadia Podoroska 6-2, 6-3 at Ken Rosewall Arena.

Underdog Billy Harris looked on track to seal the tie for Britain when he went a set up against Tomas Martin Etcheverry.

But the Argentine, fresh off a one-sided season-opening loss to Australia's Alex de Minaur at the weekend, rallied for a gutsy 3-6, 6-3, 6-2 win to send the tie to a deciding mixed doubles.

Etcheverry fronted up again with Maria Carle to take on Boulter and Charles Broom, with the British pair coming out on top 7-6 (7/4), 7-5.

"Very, very pleased with today. We played so great together considering it was our first time. So thrilled to get the win," Boulter said.

World number two Iga Swiatek kicks off her season in Sydney later when Poland face Casper Ruud's Norway.

C.Kreuzer--VB