World number one Novak Djokovic will open his 2024 season at the mixed teams United Cup in Australia, but injury-plagued Rafael Nadal was not on the Spain team announced Friday.

Djokovic will spearhead Serbia at the tournament from December 29-January 7 in Sydney and Perth as he builds towards the defence of his Australian Open title.

Nadal was part of the Spain team in 2023 before injuring his hip at the opening Grand Slam of the year. He has been out of action since and underwent surgery in June.

Tennis Australia chief Craig Tiley said this month Nadal planned to make his return to Grand Slam tennis at the Australian Open, which follows the United Cup.

Nadal responded on social media by thanking Tiley for his "confidence", without confirming if he would appear.

There are ATP events in Brisbane, Adelaide and Auckland in early January that Nadal could also use to prepare for the Australian Open.

Nine of the world's top 20 men and five of the top 10 women will feature at the United Cup, with defending champions the United States among the favourites, led by Jessica Pegula and Taylor Fritz.

Poland, spearheaded by world number two and four-time Grand Slam winner Iga Swiatek, were named top seeds for the 18-nation event.

Other big names to sign on include Stefanos Tsitsipas, Casper Ruud, Alexander Zverev, Maria Sakkari, and Caroline Garcia.

"They are all set to play in a format designed to showcase both the men's and women's game and the unique equality in tennis," said tournament director Stephen Farrow.

Countries will be drawn into six groups of three and compete in a round-robin format. Each tie will feature one men's and women's match as well as a mixed doubles clash.