PERTH, Dec. 31, 2018 (Xinhua) -- Stefanos Tsitsipas of Greece returns the ball during the men's single match against Frances Tiafoe of the United States between the United States and Greece at Hopman Cup mixed teams tennis tournament in Perth, Australia, Dec. 31 , 2018. Stefanos Tsitsipas won 2-1. (Xinhua/Zhou Dan/IANS)morePERTH, Dec. 31, 2018 (Xinhua) -- Serena Williams of the United States returns the ball during the women's single match against Maria Sakkari of Greece between the United States and Greece at Hopman Cup mixed teams tennis tournament in Perth, Australia, Dec. 31 , 2018. Serena Williams won 2-0. (Xinhua/Zhou Dan/IANS)morePERTH, Dec. 31, 2018 (Xinhua) -- Serena Williams of the United States returns the ball during the women's single match against Maria Sakkari of Greece between the United States and Greece at Hopman Cup mixed teams tennis tournament in Perth, Australia, Dec. 31 , 2018. Serena Williams won 2-0. (Xinhua/Zhou Dan/IANS)morePERTH, Dec. 31, 2018 (Xinhua) -- Maria Sakkari of Greece returns the ball during the women's single match against Serena Williams of the United States between the United States and Greece at Hopman Cup mixed teams tennis tournament in Perth, Australia, Dec. 31 , 2018. Maria Sakkari lost 0-2. (Xinhua/Zhou Dan/IANS)more