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 (£640,000) bonus.
Her appearance in Sunday's final combined with a win last week in Toronto has put Williams at the top of the US 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 (£2.3 million), with the million-dollar bonus supplementing the $2.6 million (£1.66 miilion) cheque for winning the women's singles at the US Open.
Nadal to meet Isner in Cincinnati final
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 Belarusian world number two Victoria Azarenka, who was a 4-6 6-2 6-3 winner over 2009 champion Jelena Jankovic of Serbia.
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.
While Williams and Li may have struggled with their serves, their problems were minor compared to Azarenka and Jankovic, who managed to hold serve a grand total of just four times over three ugly sets.
Of the 27 games played, 23 ended in breaks, with the miserable serving display marred by 20 double-faults, 11 by Azarenka and nine from Jankovic.
"I don't care about that," said Azarenka, when asked about the incredible number of breaks. "As long as I won the match, that's all that matters, and I didn't count.
"I noticed it was a lot of breaks but who really cares? I think everybody kind of struggles with a serve here for some reason, but whatever."
"I definitely would not have predicted myself being in the final here at all," said Williams, who will be chasing a career-high ninth title in a single season on Sunday.
"I came in here not playing my best and then now I'm in the final. It's just almost weird.
"It was definitely a little scratchy but I was able to get through. I just tried to stay positive.
"When I was down, I just got really relaxed and I felt like, you know, Serena, you don't really have much to lose. So just kind of relax and get through it."
Stosur splits with coach
Williams storms into last four