Williams was especially sharp in the second half of the year going 31-1 and winning at Wimbledon, Stanford, Calif., the Olympics, the U.S. Open and the WTA Championships. She lost three sets at Wimbledon but only one -- in the U.S. Open finals to world No. 1 Victoria Azarenka -- the rest of the year.
She opened the year winning 10 of her first 12 matches and then took 17-of-18 -- losing in the first round at the French Open -- before the stunning second-half run.
"I've always said if I'm playing well and doing everything right, it's really difficult to beat me," Williams said at the WTA Championships. "I still believe that, which is great that I can still kind of play that way.
"I feel like there are always ways for me to improve, but I think it's a true statement, without trying to sound really full of myself or anything."
It is the fourth time Williams has been voted Player of the Year. She also won in 2002, 2008 and 2009. Only Steffi Graf (eight) and Martina Navratilova (seven) have won the honor more times.
The WTA Player of the Year Award is determined by a vote of international tennis media members.