Second seed Li, who won the Australian Open earlier this year, has a WTA-leading 21 match wins so far this year heading into the showdown against Williams.
Williams booked her place in the final thanks to a 6-4 6-3 victory over Maria Sharapova earlier on Thursday.
“I have always felt when I’m playing at my best, then it’s hard for people to beat me,” said Williams, who is seeking a record seventh Key Biscayne title.
MORE: The latest football news
MORE: The latest tennis news
BIOGRAPHY: Marcus Rashford