Defending event champion Maria Sharapova has withdrawn from the Brisbane International due to injury.

The five-time grand slam winner announced her withdrawal just hours before she was due to take the court for her opening round match against Ekaterina Makarova on Pat Rafter Arena.

The 28-year-old said she had a left forearm injury and it was in her best interests to withdraw and ensure she is fit for this month’s Australian Open.

Sharapova will be replaced in the draw by a lucky loser, who will play Makarova for a second round berth against Germany’s Andrea Petkovic.

Sharapova’s withdrawal comes just a day after world No.1 and defending Australian Open champion Serena Williams skipped her opening match of the Hopman Cup in Perth due to knee soreness.

The world No.4 missed last year’s US Open due to injury but said last week she was feeling 100 per cent ahead of her title defence in Brisbane.

Sharapova’s withdrawal overshadowed the on-court action on Tuesday, with Italian eighth seed Roberta Vinci cruising into the quarter-finals.

Vinci eliminated Slovakia’s Dominika Cibulkova 6-1 6-1 in just over 70 minutes to set up a quarter-final clash with either top seed Simona Halep or two-time Australian Open champion Victoria Azarenka.