Gao Xinyu (L) of China shakes hand with Beatriz Haddad Maia of Brazil after their women's singles match in the Group E round robin match between China and Brazil at the 2025 United Cup tennis tournament in Perth, Australia, Dec. 27, 2024. (Xinhua/Ma Ping)

Gao Xinyu came back from one set down and Zhang Zhizhen was almost flawless as China started the mixed team United Cup with a 3-0 victory over Brazil in Perth on Friday.

China had a 2-0 lead ahead of the mixed doubles after Gao and Zhang prevailed in contrasting singles matches at RAC Arena.

Fifth-seeded China's chances of a deep run in the tournament appeared to take a major hit when women's world No. 5 Zheng Qinwen pulled out as she prioritized rest ahead of the Australian Open.

But the 175th-ranked Gao filled her shoes brilliantly after bravely overcoming cramp to edge Beatriz Haddad Maia 5-7, 6-4, 7-5 in a three-hour, 22-minute thriller that was the longest women's singles match in the tournament's history.

Gao required a medical timeout in the second set and had her left thigh heavily strapped before the 17th-ranked Haddad Maia also succumbed to cramp during a gripping third set and resorted to several underhand serves.

Haddad Maia moved better after that, but Gao was not to be denied and clinched the biggest victory of her career in her first match against a top 50-ranked player.

"I have no idea what happened, my brain was empty and my whole body was cramping," Gao said. "It's a really big stage and it was a high quality match. I just pushed myself."

It was far easier for the 45th-ranked Zhang, who thrashed Thiago Monteiro 6-3, 6-0 in a 54-minute demolition. Zhang dictated rallies from the back of the court and completely overwhelmed his 109th-ranked opponent to ensure China sealed the Group E tie.

"Yes, because of my team, they're doing a very good job," Zhang said when asked if he was a better player than 12 months ago.

He went on to prevail in the mixed doubles together with Zhang Shuai, beating Brazil's Carolina Alves and Rafael Matos 6-4, 7-5.

Earlier, Kazakhstan launched the tournament and the new tennis season with a tough 2-1 victory over Spain in Group C action.

Spain took the opening victory after former world No. 10 Pablo Carreno Busta thrashed Alexander Shevchenko 6-2, 6-1 in 64 minutes.

The pressure was on world No. 6 Elena Rybakina, who responded with a convincing 6-2, 6-3 victory over Jessica Bouzas Maneiro. She jumped out of the blocks with a 4-0 lead and cruised to victory despite Bouzas Maneiro putting up a fight.

It was a strong start to Rybakina's quest of adding a second Grand Slam trophy to her memorable victory at Wimbledon in 2022, where she became the first player representing Kazakhstan to win a Grand Slam title.

Rybakina was runner-up to Aryna Sabalenka at the 2023 Australian Open, but couldn't replicate that success earlier this year at Melbourne Park after crashing to a shock second-round exit at the hands of Anna Blinkova.

"It was a really tough one, first matches are always difficult," Rybakina said. "I'm really happy with the way I played today. I'm focusing on the Grand Slams and trying to get some big titles."

Rybakina and Shevchenko clinched a thrilling 7-6 (4), 6-7 (2), 10-7 victory in the deciding mixed doubles over Carreno Busta and Yvonne Cavalle-Reimers.

The action in Sydney will start on Saturday with France playing Switzerland before host Australia takes on Argentina.

Croatia faces Canada on Day 2 in Perth, while men's star Stefanos Tsitsipas leads Greece against Spain.