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

The top seed and three-time champion at Crandon Park must get to the final to be certain of remaining atop the WTA table. If she loses Thursday to the winner from her sister Venus or Czech Iveta Benesova, then Russian Dinara Safina 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 beat her last year in Stuttgart.

"I just had a really slow, retarded start," complained the American.

"Maybe I was tired, but it's still not an excuse, really. I definitely wasn't moving my feet at all. I just wasn't doing anything that made a lot of sense. I'm a little disappointed. I don't think I played great, but I'm glad to have gotten through."

The powerful Williams was able to turn the tables as Li faltered, with the seed running it out in the final set despite a total of 37 unforced errors and conversions on just four of 10 break points.

"I definitely played better in the third set, which was good," said Williams, after defeating her third Chinese opponent in as many matches. "I needed to play pretty well to win the match."

Li was unable to close out the potential upset after her fast start and lamented her own eight double-faults in the nervous final set.

"She was so strong on the court. I had chances in the second set, maybe six or seven breakpoints, but I didn't hold for one point," Li said. "She didn't gave me any chance."

2007 men's winner Novak Djokovic booked his place in the semi- finals as he finally overcame Jo-Wilfried Tsonga with a 6-3, 6-4 defeat of the 10th-seeded Frenchman. Djokovic won the 2008 Australian Open against Tsonga, then lost their next four encounters.

The Serb, whose only title this season came at Dubai, improved to 21-7 in 2009. (dpa)