Serena breezes past Azarenka
American seals semifinal berth
The Wimbledon and US Open champion recorded a 6-4, 6-4 win at the season-ending event, her fifth triumph over the Belarusian this year. Williams also owns an 11-1 record in their career head-to-heads.
The win was Williams' third at the tournament and while she is guaranteed to top the round robin Red group, Azarenka must now beat China's Li Na on Friday to reach the last four.
The eighth-ranked Li ended German Angelique Kerber's participation in the event with a 6-4, 6-3 triumph. Italy's Sara Errani is also still in semifinal contention after a 6-3, 2-6, 6-0 win over Australian Samantha Stosur.
Errani faces a must-win match against Poland's Agnieszka Radwanska if she is to join world number two Maria Sharapova in advancing from the White group.
Williams has won 12 of the 13 sets she has played with Azarenka in 2012. Her earlier wins over the 23-year-old came at Madrid, Wimbledon, the Olympics and the US Open.
Azarenka's power troubles nearly every other player on the WTA Tour but Williams has a substantially bigger serve than most and an equally good return.
Williams was again let down by her serving, but she still managed 35 winners to just 15 for Azarenka, broke her five times and won the match when Azarenka doubled faulted for the ninth time.
"I had too many opportunities that I didn't use," Azarenka said. "She played really well. I didn't execute well enough. For me, the key word today was discipline, which was missing."
Almagro stays in hunt
Sixth seed Nicolas Almagro stormed through the deciding set to defeat Sam Querrey 6-4, 6-7, 6-0 and seal a place in the Valencia Open quarterfinals on Thursday, boosting his chances of making next month's World Tour finals in London.
Almagro broke unseeded American Querrey's serve three times in the final set and will play top seed David Ferrer for a place in Saturday's semis.
Ferrer beat Albert Ramos in an all-Spanish second-round match 6-4, 6-2. Defending champion Marcel Granollers made it through to the quarterfinals when he came from a set down to beat Frenchman Gilles Simon 6-7, 6-3, 6-2.
Granollers will play Alexandr Dolgopolov after the Ukrainian saw off Gilles Muller of Luxembourg 6-4, 6-4. The winner will face either Austria's Jurgen Melzer, who beat Xavier Malisse 7-5, 6-2 or Malisse's Belgian compatriot David Goffin in the last four.
World number four Rafael Nadal announced on Thursday he had pulled out of the Tour finals as he recovers from a knee injury that has sidelined him since Wimbledon in June.
Nadal's withdrawal secured Juan Martin del Potro's place in London and there are two berths still up for grabs.