Australian Open 2016: Serena Williams quits warm-up event

Serena Williams has won 21 Grand Slam titles

Serena Williams has withdrawn from a warm-up tournament before this month's Australian Open because of injury concerns.

World number one Williams retired with a knee injury when trailing Australia's Jarmila Wolfe 7-5 2-1 in the Hopman Cup on Tuesday.

She had been due to play in the United States' final match of the round-robin tournament on Thursday.

Maria Sharapova has withdrawn from the Brisbane International this week.

The five-time Grand Slam winner retired with an arm injury, while world number two Simona Halep pulled out with an inflamed Achilles and world number three Garbine Muguruza quit the tournament with a foot injury.

The Australian Open begins on 18 January.

Williams, who also missed the United States' opening Hopman Cup tie on Monday, will be replaced by Vicky Duval against the Czech Republic.

The 34-year-old, who will be attempting to retain her title in Melbourne, has a recurrence of a problem that sidelined her for much of the latter part of 2015.

She trained briefly on Tuesday and took to the court against Wolfe, but appeared very restricted in her movement, especially on her left side.

Williams served for the first set, but was broken by Wolfe, who then took the set 7-5.

When Wolfe broke her again for a 2-1 lead in the second set, Williams called for the trainer and retired after a brief discussion.

Russian Sharapova, world number four, was hoping to defend her title in Brisbane in what was her only scheduled event before the Australian Open.