World number one Williams continued her utter domination of Maria Sharapova with a 6-4 6-3 win over the fourth-seeded Russian, while second-ranked Li won a rematch of her Australian Open final, beating Dominika Cibulkova 7-5 2-6 6-3 to maintain her perfect record against the Slovakian.
A six-time Miami champion, the American's victory ran her winning streak against Sharapova to 15 matches, a stunning run that stretches back to 2004.
Li's mastery over Cibulkova may not span as many years but is no less complete, the Chinese veteran sweeping all seven of their meetings over a decade-long span.
Three of those wins have come this year, with Asia's first grand slam singles champion defeating the Slovakian in straight sets at Melbourne Park, in three sets at Indian Wells two weeks ago and now at Miami.
"Good challenge. The last time I played her (Williams) was in the (WTA Tour) Championships so it has been four months and this time we really can see how I have improved," Li told reporters as she looked ahead to the final.
"I am really happy I can play her again. Everyone will be confident if they come to the final. I think for sure it will be a tough match."
The 10th seeded Cibulkova had looked ready to deny tennis fans a marquee final between the best two players in the world when she stormed back to take the second set and opened the third with a break to forge a 3-1 lead.
However, Li kept her composure and swept the next five games to cap an enthralling fightback and her spot in the final.
Sharapova and Williams, who have both achieved a career grand slam and held the number one ranking, were expected to develop into one of the great rivalries in women's tennis.
But it has not quite worked out that way with Williams winning 16 of the 18 meetings between the pair.
In Sharapova's 16 losses, the Russian has only managed to take two sets off the 17-times grand slam winner.
"Despite my results against her, I still look forward to playing against her because you learn so much from that type of level which she produces," Sharapova said. "There is no reason for me to have any pressure because of my results against her.
"She's an incredible champion. That's the reason she's at the top.
"She's accomplished a lot, her tennis speaks for itself and I have nothing to lose out there against her."
Proving there is no place like home, Williams, who lives an hour's drive from the Tennis Center at Crandon Park, has won the Miami event more than any other tournament, her six titles beating the five U.S. Open, Australian Open and Wimbledon grand slam wins she has amassed over the years.
A five-time runner-up in Miami, Sharapova had her chances to end the drought by grabbing an early break in both sets but could not turn the advantage into a win.
The fourth seed stepped onto a blustery centre court looking focused and determined and was rewarded with the first break to go up 4-1.
But Williams was soon back in control breaking Sharapova twice as she swept through the next five games to take the opening set.
Sharapova also opened the second set with an early break to go 2-0, but again the 32-year-old American was ready, answering right back with a break of her own.
The Russian held serve just once more before Williams moved into high gear, tearing through the last four games and putting an exclamation point on the contest by closing out the match with another break when Sharapova slammed a forehand return into the net.
"I didn't think I would be sitting here the way I was playing in the beginning of the week, so I'm excited to still be in the tournament," Williams said.
"I definitely feel better than what I did in my first couple of matches but I also feel there is a big gap for me to play even better and to be more consistent and get to the level that I was playing at a little bit last year."