Miami  - Novak Djokovic turned the corner in his career matchups with Jo-Wilfried Tsonga, winning 6-3, 6-4 on Wednesday to reverse a run of four straight losses to the Frenchman and book a semi-final place at the Miami Masters.

The 2007 champion also began turning around his reputation as a shirker, fighting through what he said was a stomach problem as he stayed on court to see out the victory.

"I've been not feeling well with the stomach last night and this morning, and probably the heat affected as well, combined of course with the pressure on the court and all these things," said the third seed, who beat Tsonga for the 2008 Australian Open title.

"But it's just incredible the way I stood up and tried to survive there on the court. It was really important for me to win in the straight sets and to save those service games, because I was really lucky. I played some good shots."

After Djokovic lifted Melbourne at the expense of Tsonga, he lost their next four meetings, most recently in February. The Serb, whose only title this season came at Dubai, improved to 21-7 in 2009.

"I have to be very happy with today's performance, because I have been struggling to find a way to win against him in the last four encounters," Djokovic said. "I just knew that I need to stick in and, you know, try to wait for my chances and be aggressive.

"Overall, I'm really satisfied."

Women's number 1 Serena Williams held onto her top ranking with a crucial victory over China's Li Na, 4-6, 7-6 (7-1), 6-2 to reach the last four.

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," the US player complained.

"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." (dpa)