Coco Gauff and Taylor Fritz will spearhead the United States' title defence at the mixed teams United Cup in Australia, organisers said on Thursday, with Iga Swiatek leading Poland's quest for revenge.
World No 3 Gauff and sixth-ranked Fritz edged Poland to win the title at the season-opening ATP-WTA event in January, a key lead-up tournament to the Australian Open.
Gauff beat world No 2 Swiatek in the decider, while Fritz downed Hubert Hurkacz, who is also returning.
Five of the world's top 10 men and four of the world's top 10 women are among those signed up for the January 2-11 tournament which will see 18 countries playing in Sydney and Perth.
Each team features three men and three women, with ties comprising one men's and one women's singles and an often decisive mixed doubles.
Australian Open champion Jannik Sinner and rival Carlos Alcaraz have opted for an exhibition match in South Korea on 10 January before making their way to Melbourne Park for the season-opening Grand Slam starting eight days later.
Serbian great Novak Djokovic is yet to reveal his early season plans.
Defending women's Australian Open champion Madison Keys is signed up for the Adelaide International from 12-17 January, alongside Olympic gold medallist Belinda Bencic.
World No 1 Aryna Sabalenka, who Keys stunned in the Australian Open final, will start her 2026 campaign at the Brisbane International as she searches for a fifth Grand Slam title.
"The entry list for the United Cup this summer is world class," said tournament director Stephen Farrow.
"We look forward to the official draw next Monday to determine where our 18 countries will play across Perth and Sydney.
Germany will be guided once again by world No 3 Alexander Zverev alongside newcomer Eva Lys in pursuit of a second United Cup after victory in 2024.
Japan make their debut with four-time Grand Slam winner Naomi Osaka lining up with 2019 junior Wimbledon champion Shintaro Mochizuki.
Hosts Australia are led by Alex de Minaur, Stefanos Tsitsipas and  Maria Sakkari are back for Greece, while Jack Draper and Emma Raducanu lead the British charge, and Jasmine Paolini headlines the Italian team.