Former Wimbledon champion Elena Rybakina helped lead Kazakhstan to their first-ever United Cup tie win on Friday.

Kazakhstan triumphed 2-1 against Spain on the opening day of the United Cup, moving them into top spot in Group C.

Alexander Shevchenko lost his singles match 2-6 1-6 to Pablo Carreno Busta, who became the lowest-ranked player (196) to win the opening match of a tie at the United Cup.

However, Rybakina gained a win in her first career meeting with Jessica Bouzas Maneiro, cruising past the Spaniard in straight sets before teaming up with Shevchenko to clinch the mixed doubles.

It was a tightly fought encounter that went down to the wire, with Carreno Busta and Yvonne Cavalle-Reimers pushing them all the way, but Kazakhstan showed their mettle, holding out to win 7-6 (7-4) 6-7 (2-7) 10-7.

"It was a really tough one, first matches are always difficult," Rybakina said in her on-court interview.

"It's not easy to get used to [it] that quick, but I'm really happy with the way I played."

Meanwhile, despite very different wins in their singles matches, China rallied to a perfect start in Perth, beating Brazil 3-0 in Group E.

Gao Xinyu claimed a major upset in her opener, coming from behind to overcome Beatriz Haddad Maia 5-7 6-4 7-5 in an epic across three hours and 22 minutes, the longest WTA singles match in the competition's history.

Gao, a replacement for gold medallist Zheng Qinwen, had never faced a top 50 opponent before, with her only victory against a player ranked in the top 100 coming back in 2017.

"I have no idea what's going on right now," said Gao in the on-court interview.

"My brain was empty, and my whole body was cramping."

Zhang Zhizhen then coasted past Thiago Monteiro in 54 minutes to ensure China took the win, before teaming up with Zhang Shuai in a winning doubles performance to round off the tie.