Miami  - Serena Williams retained a grip on her top ranking with a crucial victory over China's Li Na, 4-6, 7-6 (7-1), 6-2 to reach the semi-finals of the Miami Masters on Wednesday.

The top seed and three-time champion at Crandon Park must get to the final to ensure that she remains on atop the WTA table. If she loses on Thursday to the winner from her sister Venus or Czech Iveta Benesova, then Russian Dinara Safina takes steps up to the summit.

Williams, playing in only her fifth event of the season, put on a horror-show first set, dropping to 0-5 against China's number 40 who neat her last year in Stuttgart.

But the powerful American was able to turn the tables as the Chinese faltered, with Williams running it out in the final set despite a total of 37 unforced errors and conversions on just four of ten break points. (dpa)