The Netherlands won the decisive mixed doubles match to clinch its opening United Cup group stage tie against Norway 2-1, while China swept the Czech Republic 3-0 on Saturday.

Mixed teams from 18 countries are playing in Perth and Sydney in what doubles as a warm-up for the Australian Open. The format is a men's and women's singles match and a mixed doubles.

In Sydney, Arantxa Rus gave the Dutch an early lead with a 7-6 (4), 6-1 victory over Malene Helgø, before No. 11 Casper Ruud levelled the tie for the Norwegians by beating Tallon Griekspoor 6-3, 6-4.

Dutch pair Demi Schuurs and Wesley Koolhof won the decisive mixed doubles match against Ruud and Ulrikke Eikeri 7-6(5), 7-5 to take the Group F tie.

In the Group E match in Perth, China raced to a 2-0 lead after Zhang Zhizhen rallied to beat Jiri Lehecka 5-7, 6-3, 6-4 and Zheng Qinwen upset No. 7 Marketa Vondrousova 6-1, 2-6, 6-1. The Chinese pair then sealed a 3-0 clean sweep of the tie by winning the mixed doubles.

Former Australian Open champion Angelique Kerber returned to the court after giving birth in February in a 6-4, 7-5 defeat to Italy's Jasmine Paoline in Germany's opening Group D match in Sydney.

Kerber is joined in the German team by No. 7 Alexander Zverev, who faces Lorenzo Sonego in an evening match.

Later Saturday in Perth, women's No. 1 Iga Swiatek takes to the court for Poland's opening Group A match against Brazil. Swiatek faces No. 11 Beatrice Haddad Maia before Hubert Hurkacz plays Thiago Seyboth Wild in the men's match.

Brazil lost its opening match against Spain 2-1 on Friday.

On Sunday, men's No. 1 Novak Djokovic returns to Perth for the first time since the Hopman Cup in 2013 when he leads Serbia in its first group stage match against China.

Djokovic won three Grand Slam singles titles this year, missing out on only Wimbledon. He's preparing to try and win the Australian Open for a record-extending 11th time.