A fan supporting Serena Williams has caused controversy by appearing to "wear blackface" for an Australian Open match in Melbourne.

A picture of the fan sporting black facial makeup and holding a sign declaring "Keep calm and be Serena" has gone viral on social media following the semi-final match, in which Williams defeated Agnieszka Radwanska, on Wednesday.

Twitter users responded with outrage over the picture, with one writing: "It's straight bonkers they let her in. They better have kicked her out."

Another said: "I see no fan. I see an ignorant, disrespectful, fool. Seriously? Is this not 2016?"

Williams went on to lose to Angelique Kerber in the final of the Slam on Saturday.