After dropping the first set, Williams survived a nine deuce game to hold serve before breaking serve then holding serve again to jump ahead in set to 3-0.
With Williams serving up 5-3 on setpoint number three, a Li inside out forehand down the line landed wide to level the match at one set all.
In the final set, Williams took another 3-0 lead by breaking serve at 15-40 after striking a backhand down the line winner.
Williams had struggled physically in her semifinal with fatigue but was able to turn the match around in the third set which appeared to be the case in the final round match as well.
The American blasted an ace out wide serving at 40-30 to reach 4-0.
Two Li netted forehand volleys down the line followed by a double fault on breakpoint set Williams ahead 5-0.
Li reached breakpoint four times as Williams served for the match. At 15-40, a crosscourt backhand from Li landed wide. At 30-40, Williams hit an inside out backhand down the line winner.
The last two breakpoints of the match were both erased with Williamsí aces number five and six both hit out wide.
On match point number two for Williams, the American pounded a backhand down the line winner to end the match.
In addition to her nine WTA Tour titles won this year, Williams also took the grand slam titles at the French Open as well as in her home country at the US Open in route to a WTA Tour record $12,385,572 earned in prize money this year.