Gao Xinyu will face the formidable challenge of replacing women's world No. 5 Zheng Qinwen when she partners with Zhang Zhizhen, China's highest-ranked men's player, at the season-opening United Cup starting on Dec. 27.

Fifth-seeded China opens its tournament against Brazil on Friday at the 15,000-seat RAC Arena in Perth. Defending champion Germany, led by men's world No. 2 Alexander Zverev, is also in Group E.

Zhang Zhizhen of China competes during the men's singles second round match between Zhang Zhizhen of China and Thiago Agustin Tirante of Argentina at the French Open tennis tournament at Roland Garros in Paris, France, on June 1, 2023. (Xinhua/Gao Jing)

Organized by the ATP and WTA, the third edition of the mixed team event in Sydney and Perth effectively launches the 2025 international tennis season ahead of the Australian Open, which is annually the first Grand Slam on the calendar.

Seven of the world's top 10 women's players and six of the highest 12 ranked men will feature at the United Cup, culminating with the final on Jan. 5, 2025, in Sydney.

China's hopes of improving on its quarterfinal result in the previous edition took a major hit after Zheng withdrew to focus on rest before next month's Australian Open.

Zheng is coming off a breakout season, highlighted by winning the women's Olympic gold medal. She was runner-up to current world No. 1 Aryna Sabalenka at Melbourne Park after playing in the United Cup in Perth last season.

She was set to again represent China before pulling out.

"After the long season that 2024 was for me, I need a few extra weeks of rest, recovery, and good training to get ready for the new season," Zheng wrote on social media. "I had such a fantastic time at the United Cup in January of this year, and therefore will miss the event greatly."

Zheng's withdrawal gives an opportunity to world No. 175 Gao. The 27-year-old most recently reached the round of 32 at the Hong Kong Open. She will be the underdog against 17th-ranked Beatriz Haddad Maia of Brazil before concluding the group stage against Germany's Laura Siegemund.

China's hopes for a deep run in the tournament will likely rest with men's 45th-ranked Zhang, who reached the third round of this year's French Open. The 28-year-old won a silver medal at the Paris Olympics in mixed doubles alongside Wang Xinyu.

He also enjoyed a barnstorming semifinal run at this year's Australian Open doubles with Czech Tomas Machac.

Zhang will face 109th ranked Thiago Monteiro in the United Cup, before taking on Zverev on Dec. 30. Zverev will be seeking to sharpen his Australian Open preparations as he aims to end his Grand Slam title drought.

Eighteen teams are drawn into six groups in a round-robin format, with the group winners and two best-performing runners-up advancing to the quarterfinals.

Top-seeded United States is headlined by women's world No. 3 Coco Gauff, who did not win a Grand Slam this year after claiming the 2023 U.S. Open. She partners with men's No. 4 Taylor Fritz, who reached his first major final at the U.S. Open before losing to world No. 1 Jannik Sinner in straight sets.

The United States is in Group A alongside Canada and Croatia.

Women's world No. 2 Iga Swiatek will draw particular attention as she returns following a one-month suspension for testing positive for the banned substance trimetazidine.

The five-time Grand Slam champion will spearhead second-seeded Poland with Hubert Hurkacz, currently No. 16 in the men's rankings. They guided Poland to a runner-up finish in the last United Cup. This time, Poland has been drawn with the Czech Republic and Norway in Group B.

Stefanos Tsitsipas, a former Australian Open finalist, begins his quest for a maiden Grand Slam title as the talisman for third-seeded Greece, which faces Kazakhstan and Spain in Group C. Spain will be without star Carlos Alcaraz, turning to former top-10 player Pablo Carreno Busta to lead its charge.

There will be intrigue around women's fourth-ranked Jasmine Paolini, who reached the final of this year's French Open and Wimbledon. She partners with Flavio Cobolli for Italy, which meets France and Switzerland in Group D.

Host Australia's fortunes rest largely on Alex de Minaur, the country's top-ranked men's player. He will face his fiance Katie Boulter when Australia meets Britain, just days after they became engaged.

Argentina is also in Group F.