Williams, who has been critical of her performance in Cincinnati despite making it to the final for the first time, was not at her dominating best against Li but it was a good enough effort to put the American in line for a $1 million bonus.
Her appearance in Sunday's final combined with a win last week in Toronto has put Williams at the top of the U.S. Open Series bonus challenge and sends her to Flushing Meadows with a chance to compete for the largest payout ever awarded at the year's final grand slam.
Williams can pocket $3.6 million, with the million-dollar bonus supplementing the $2.6 million cheque for winning the women's singles at the U.S. Open.
In a career that has generated 54 singles titles including 16 grand slams, Williams has won just about everything there is to win in her sport but Cincinnati, one of the WTA Tour's most prestigious tournaments, is not among them.
Williams can fill that hole in her resume on Sunday when she takes on the winner of the second semi-final between world number two Victoria Azarenka and Jelena Jankovic.
Li stepped onto centre court well-rested having received a walkover into the semi-finals when Poland's Agnieszka Radwanska withdrew from the tournament to attend her grandfather's funeral.
But the 31-year-old fifth ranked Chinese got off to a sluggish start as Williams recorded the early break to move ahead 4-1.
Li hit back, sweeping through the next four games to pull in front 5-4, but Williams upped her game when needed, breaking her Chinese opponent twice more to take the set.
Williams and Li held serve to open the second but the set would soon dissolve into series of breaks.
In the end, Williams claimed the decisive one when Li double-faulted to end her Cincinnati defence and hand Williams the match.
(Editing by Ian Ransom)
