PERTH: Tennis world No 1 Serena Williams has pulled out of the mixed-teams Hopman Cup in Perth due to a knee problem. With less than a fortnight to go before her title defence at the Australian Open, Williams confirmed she would not play in the United States’ final round-robin tie against the Czech Republic on Thursday. The 34-year-old, who battled knee problems in the latter half of 2015, retired from her first singles match against Australian Jarmila Wolfe at the Hopman Cup in Perth on Tuesday. She had already missed the United States’ opening Hopman Cup tie due to inflammation of her left knee before attempting to play against Wolfe. The American retired when trailing 7-5, 2-1, having appeared extremely restricted in her movement during the match, particularly on her left side.