PERTH, Australia -- Kazakhstan reached the quarterfinals of the United Cup mixed team tournament with a comfortable win over Greece on Monday as Elena Rybakina defeated Maria Sakkari 6-4, 6-3 after Alexander Shevchenko stunned Stefanos Tsitsipas.
Former Wimbledon champion Rybakina, who is using the 18-nation tournament to prepare for next month's Australian Open, won a tight opening set against Sakkari before powering ahead 5-0 in the next set.
She was broken late but stayed composed to close the contest on serve, leaving her new coach Goran Ivanisevic beaming.
"We've just been working a couple weeks together. This is our first official tournament," Rybakina said. "We're still getting to know each other a bit. He's a fun guy. He gives some experience from when he played and also as a coach. I'm just happy to work with him. Hopefully, we'll have some good results together."
Shevchenko pulled off a major upset earlier when he downed an ill and error-prone world No. 11 Tsitsipas 6-4, 7-6 (0) to put Kazakhstan on course for victory in the Group C tie in Perth.
"It will definitely go in my record as one of my best wins," Shevchenko said. "He wasn't at his best today but still I played really well and I found the moments to win the match."
In Sydney, Katie Boulter and Charles Broom guided Britain to a 2-1 win over Argentina in Group F, winning their mixed doubles rubber against Maria Lourdes Carle and Tomas Martin Etcheverry 7-6 (4), 7-5.
Boulter put Britain up 1-0 with a 6-2, 6-3 win over Nadia Podoroska but Argentina fought back immediately as Etcheverry overcame a stomach issue to down Billy Harris 3-6, 6-3, 6-2.
World No. 2 Iga Swiatek will be in action when Poland faces Norway in Group B, while defending champions Germany meets China in Group E.
Reuters contributed to this report.
