Gao Xinyu came back from one set down and Zhang Zhizhen was almost flawless as China began 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, prioritising 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. (Xinhua)