The American was not at her best in the first three months of 2012, her exit at the fourth-round stage of the Australian Open symptomatic of her struggle to find top form.
But back-to-back tournament wins in Charleston and Madrid in April and May marked a change in fortune for the world number three.
She went on to win 48 of 50 matches, landing grand slam titles at Wimbledon and the US Open as well as Olympic gold at London 2012.
It is the fourth time Williams has won the WTA award after successes in 2002, 2008 and 2009.
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