Victoria Azarenka secured the season-ending world No.1 spot and set-up a WTA Championships semi-final with Maria Sharapova after batting past Li Na 7-6 (7/4) 6-3 in Istanbul.
Despite hitting only six winners in the duration of the match, Azarenka showed typical fight and determination to clinch victory.
The Belarussian lost her serve twice in an opening set which saw Li Na fail in her attempt to serve out the set.
The Chinese No.1 coughed up two double faults when leading 5-4 and was made to pay when Azarenka edged the ensuing tie-breaker 7-4.
Holding serve proved a tough task for both players at the start of the second set and, after three successive breaks, Li double faulted for a seventh time to gift Azarenka a 4-1 lead.
The 23-year-old was broken to love when attempting to serve out the match, however she hit back with a sixth break to eliminate Li and seal a last four date with Sharapova.
Speaking after becoming only the 11th women to end the season as world No.1, Azarenka, who won the Australian Open at the start of the year, added: "It's a dream come true, I just want to play my last two matches now and then enjoy the moment more. Right now I'm in a competition and I'm trying to focus on each day and the challenges ahead.
"It was a like a rollercoaster (today), she played well at some points and we are big shot-makers so it depends on who was gaining control. The momentum was going back and forth."