(Reuters) - American Serena Williams has been named WTA Player of the Year after one of the most successful seasons in her career.

The 31-year-old won Wimbledon and the U.S. Open to take her grand-slam haul in singles to 15 while she also won Olympic gold in singles and doubles in London.

After a relatively slow start to the year, Williams was virtually unstoppable from April to October when she won 48 of her 50 matches, culminating in the title at the WTA Championships in Istanbul.

Williams, who ended the year No.3 in the rankings but clearly the outstanding player, has now won the WTA award four times, having also taken it in 2002, 2008 and 2009.

The award is voted for by international tennis media.

(Reporting by Martyn Herman; Editing by Clare Fallon)