The 23-year-old knew victory over Na would be sufficient to seal the top spot ahead of Maria Sharapova, and the Australian Open champion overcame a difficult opponent to set up a semi-final meeting with the Russian in the last four.
“It’s a dream come true and I feel great,” Azarenka said.
“Now I just want to play my last matches of the year and then after that I can relax and enjoy this moment more.
“Right now I’m in competition and trying to focus on every single day – there’s still a lot of challenge in this tournament.”
She added: “I felt like the first set was a bit of a roller coaster – she was playing well at some points and I was playing well at some points, and we’re both big shotmakers, so it all depended on who was getting control at what time,”
“The momentum was going back and forth, but I’m glad I stayed through it and fought hard, even though I didn’t feel I was executing well.”
Azarenka beat Sharapova in three sets at Flushing Meadows in August before winning in straight sets to win the China Open, leaving the world No1 with a superior head-to-head record (7-4).
But the top seed is still expecting another gruelling encounter despite her recent dominance over the four-time Major winner.
“There are definitely not going to be any surprises tomorrow, but it’s a different occasion, a different place,” Azarenka added.
“I’ve never played her at the WTA Championships, so it’s new and it’s not new – you can take it both ways.”
Elsewhere, Agnieszka Radwanska battled from a set down to beat Sara Errani 6-7 (6-8) 7-5 6-4 to book a semi-final meeting with Serena Williams in Istanbul.
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BIOGRAPHY: Eric Bailly