Serena Williams recovered from a set down to defeat Li Na 4-6 7-6 (7/1) 6-2 and reach the semis of the Sony Ericsson Open.

The world number one made a terrible start in Miami, with her unseeded Chinese opponent racing into a 5-0 lead.

Li has struggled with a knee injury for the last year but has been in good form recently, reaching the final in Monterrey and beating in-form Vera Zvonareva in the fourth round on Monday.

And she held off the expected Williams fightback to take the first set. The second set was nip and tuck with neither player able to take advantage of their break points.

But Williams took control in the tie-break to level the match and the third set was largely one-way traffic as the top seed powered through.

In the last four she could face another meeting with her sister Venus, who takes on Iveta Benesova later tonight.