winning last year 0-6, 6-1, 6-4. Li also had defeated Venus Williams at the Summer Olympics last year in the quarterfinals 7-5, 7-5.
After Williams dropped the first five games of the match, she won four games in a row to pull to 4-5 before Li held serve to close out set one.
In set two Williams still played below her best level while Li continued to routinely power forehands as well as backhands down the lines for winners. Li held at love to reach 4 all.
A Williams doublefault set up two breakpoints for Li in the following game. However Li's forehand would falter twice with unforced errors as Williams held for 5-4.
Serving to stay in the set, a Li backhand down the line winner on gamepoint evened the set at 5 all.
Two Williams doublefaults and a backhand hit wide set up two additional breakpoints for Li. But as was her pattern for the day, Williams would turn in her best tennis when she was down.
Williams struck her first and only ace of the day to save the first breakpoint then connected with a forehand down the line winner to save the second before holding serve to pull ahead 6-5.
In the following game, two Williams groundstroke winners and a Li backhand error resulted in Williams' first breakpoint of the match. Li saved the first breakpoint with a swing forehand volley winner in her first trip to the net of the match before Williams overhit a backhand wide to send the set into a decisive tiebreaker.
But after failing to convert six breakpoints earlier in the set, Li was unable to maintain her high level of play as she would produce more errors while Williams would increasingly minimize her own.
“I gave her the tiebreak,” Li said. “I couldn’t focus in the final set. After she won the second set, I didn’t think I had a chance.”
Williams steamrolled through the tiebreaker before breaking serve twice early in the final set to completely pull away from Li near the match's close. Li netted an overpowering serve down the tee from Williams on matchpoint to end the battle after two hours and seven minutes of play.
And although she was able to perform today when she needed it the most, Williams' six doublefaults and 37 errors may not be sufficient in Thursday's semifinal round.
“The whole match is still an F,” Williams said. “I’m glad to have gotten through.”
Williams is to play the winner of tonight's quarterfinal match between the fifth seeded Venus Williams and the 26th seeded Iveta Benesova of the Czech Republic.
Venus Williams is the only active WTA Tour player to ever defeat Serena Williams in Miami. Venus has also won 23 of her last 24 matches.
Adding extra drama to the Serena Williams semifinal is the fact that Williams will lose the world number one ranking if she does not advance to Saturday's final.