Istanbul: World No.3 Serena Williams defeated China`s Li Na in the second group match of the $4.9 million WTA Tour Championships here.
The US multiple Grand Slam winner is now on a 28-1 run since June when she won her fifth title at the Wimbledon.
After an easy opening win against Germany`s Angelique Kerber, the 31-year-old had to dig deep to beat Li 7-6(2), 6-3 in a match that saw 11 breaks of serve Wednesday.
Williams was 4-1 in the first set after serving four double faults, but stormed back to take it in a tie break.
She dominated throughout the second set making two early breaks and sealed the match from her third match point.
Williams is now top of the red group with only World No.1 Victoria Azarenka of Belarus left to face. Li is still to meet both Azarenka and Kerber.