Azarenka has been atop the rankings for the last 26 weeks. She first took over the No. 1 spot after winning the Australian Open last year and yielded it to Maria Sharapova for four weeks in the summer before regaining the top spot.
Her 45 weeks overall at No. 1 is 10th most, following Caroline Wozniacki in ninth with 67 weeks.
Williams' second half of 2012 included included wins at Wimbledon, the Olympics, the U.S. Open and the WTA Championships, pulling herself to No. 3 in the rankings.
The 2013 WTA schedule kicks off this week, with all of the Top 10 players entered in one of three tournaments. World No. 4 Agnieszka Radwanska is the top seed at the ASB Classic in New Zealand and No. 7-ranked Li Na tops the field at the Shenzhen Longgang Gemdale Open in China.
The Brisbane International features the other eight Top 10 players.
Azarenka enters 2013 with 10,595 points in the rankings, with Sharapova second at 10,045 and Williams third with 9,400.
Radwanska has 7,425, which is fourth most, and Angelique Kerber comes in fifth at 5,550. No. 6 Sara Errani (5,100 points), seventh-ranked Li (5,095) and No. 8 Petra Kvitova (5,085) are within 15 points of each other. Samantha Stosur (4,135) is ninth and Wozniacki (3,765) completes the Top 10.