The Hopman Cup has announced its full lineup of 2019 teams, and, in a surprising change from the last two years, the United States will be represented by Serena Williams and Frances Tiafoe.

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To make things even more interesting, the Americans are scheduled to play the defending champions—Roger Federer and Belinda Bencic's Switzerland—on the evening (Perth, Australia, time) of Jan. 1.

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The tournament will likely be Williams' first appearance since the US Open, with the 23-time Slam champion having already shut down her 2018 season.

The USA and Switzerland are in Group B, which also includes the new Greek team of Stefanos Tsitsipas and Maria Sakkari ...

And Great Britain's Katie Boulter and Cameron Norrie.

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Group A includes France, with Alize Cornet and Lucas Pouille ...

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Spain, with Garbine Muguruza and David Ferrer ...

Germany, with Alexander Zverev and Angelique Kerber ...

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and Australia, with Ashleigh Barty and Matthew Ebden.

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The Hopman Cup begins on Dec. 29, with Team USA playing its first match on Dec. 31.