Serena... knee trouble.

Serena Williams has withdrawn from the Hopman Cup because of a knee injury and will not play in the USA’s final match today.

The world number one missed the first match of the round-robin tournament and then retired during the second set of her clash against Australia’s Jarmila Wolfe on Tuesday.

Williams revealed she was struggling with inflammation of her left knee but did not immediately rule herself out of today’s clash with the Czech Republic.

However, the tournament announced on its Twitter feed yesterday that Williams had indeed withdrawn and would be replaced by Vicky Duval.

The injury must be a concern for Williams with less than two weeks to go until she begins the defence of her Australian Open title.

The 34-year-old brought an early end to her 2015 season partly because of pain in her knees after seeing her Grand Slam dreams ended by Roberta Vinci in the US Open semi-finals.

Meanwhile, Germany won their first match of the competition with a 2-1 victory against France.

Caroline Garcia defeated Sabine Lisicki but teenager Alexander Zverev pulled Germany level with victory over Kenny De Schepper and the German duo won the doubles.

Other result: Great Britain vs Australia Green 1-2.