Top-ranked Novak Djokovic beat Zhang Zhizhen, world No. 58, 6-3, 6-2 in 72 minutes on Sunday as Serbia edged China 2-1 at the United Cup, while the United States started their title defense with a comeback victory over Britain.
Djokovic, who eyes a record-extending 11th trophy at the Australian Open, said after the Group E tie, "I was a little bit rusty at the beginning. I was out of the tempo, rhythm for maybe the first five, six games."
Novak Djokovic (1st, L)/Olga Danilovic(2nd, L) of Serbia pose for photograph with Zheng Qinwen (2nd, R)/Zhang Zhizhen (1st, R) of China ahead of their mixed doubles of the Group E match between China and Serbia at the 2024 United Cup in Perth, Australia, Dec. 31, 2023. (Photo by Zhou Dan/Xinhua)
"He (Zhang) was serving well, and he had nothing much to lose," Djokovic added.
In the women's singles, Zheng Qinwen, world No. 15, kept China alive by beating world No. 119 Olga Danilovic 6-4, 6-2.
The 21-year-old Zheng continued her impressive start to the season, having stunned Wimbledon champion Marketa Vondrousova in China's 3-0 win over the Czech Republic in their group opener on December 30, 2023.
"I was under pressure needing a win, but I had a great performance and I'm happy with myself," Zheng said.
Following her singles win, Zheng partnered with Zhang to battle Danilovic/Djokovic in a mixed doubles match, but registered a narrow 6-4, 1-6, 10-6 loss just before midnight.
In an earlier tie in Perth, Britain's Katie Boulter shocked world No. 5 Jessica Pegula to claim the biggest win of her career, but Britain fell to defending champions the United States 2-1 in the end.
Boulter, world No. 56, staged a remarkable comeback from a set and double breakdown to win 5-7, 6-4, 6-4. It was the 27-year-old's first victory over a top-five opponent in her career.
But American world No. 10 Taylor Fritz secured a 7-6 (5), 6-4 win over Cameron Norrie, who had beaten Australia's Alex de Minaur in three sets on the opening day of competition.
"It's always a battle with Cam and it got pretty close at the end of the first set, but in the second I just hung in there with my serve," Fritz said. "I felt really sore and I overdid it running for a drop shot, but I got through."
Fritz did have some concerns after struggling with a quad strain, but he recovered to partner Pegula in the mixed doubles as they beat Boulter and Neal Skupski 1-6, 7-6, 10-7.
In the only tie in Sydney on day three, Canada started their campaign in style with a 2-1 victory over Chile in Group B.
Canada's Leylah Fernandez, a US Open finalist in 2021, eased past Daniela Seguel 6-2, 6-3 in 73 minutes.
The 21-year-old Fernandez, ranked 35th, dominated the clash and broke five times.
"First match of the season...had some nerves, but happy with the way I was able to handle it," she said.
Without their men's ace Felix Auger-Aliassime, Canada's 314th-ranked Steven Diez fell to world No. 19 Nicolas Jarry 7-5, 6-4.
But Fernandez stepped up in the decisive mixed doubles and teamed up with Diez to overcome Seguel and Tomas Barrios Vera in a three-set thriller 7-5, 4-6, 10-8.
There was no night session in Sydney due to the city's famous New Year's Eve fireworks.
Day four action will see women's world No. 1 Iga Swiatek spearhead Poland's quest for a second straight victory in the event when they face Spain, while the United States will meet host Australia in the later tie in Perth.
In Sydney, Croatia will play Norway while France is taking on Germany