Stosur, a runner-up at the French Open, played some of her most inspired tennis this year. The Australian used a powerful forehand and strong serve to rattle the normally consistent Wozniacki.
Stosur finished a tight first set with a forehand winner and turned up the heat in the second, racing to a 5-2 lead. But Wozniacki battled back and nerves seemed to get the best of Stosur as she twice double-faulted before closing out the final game when Wozniacki returned her serve long.
"I'm really pleased to get through," a beaming Stosur said as she came off the court. "It's so exciting to be here. It's a great way to finish the year. You give everything you got for the last week and it's all come together so I'm very happy."
Wozniacki, who won her opening match Tuesday against Elena Dementieva, would have secured the top ranking for 2010 with a victory Wednesday.
But she was uncharacteristically erratic, committing 17 unforced errors and failing to convert all six of her break-point opportunities. She threw her racket at one point and had several heart-to-heart talks with her father courtside.
"For me today, I didn't take my chances," Wozniacki said. "I lost the match and I need to just try to get over it fast and prepare for tomorrow."
Wozniacki, who replaced the injured Serena Williams at No. 1 earlier this month, said the thought of losing that title to second-ranked Vera Zvonareva never entered her mind. Zvonareva must reach the final to have any chance at finishing the year at No. 1.
"I don't think about it," Wozniacki said. "I go into the match every time and I want to win I want to do my job and that it. I don't really think about everything else."
Earlier Wednesday, Zvonareva claimed her second win in the White Group by beating Victoria Azarenka of Belarus 7-6 (4), 6-4 to move into a good position to reach the semifinals.
The top two players from each round-robin group will go through to the semifinals.
Zvonareva, a Wimbledon and U.S. Open runner-up, recovered from 2-0, 4-2 and 5-4 down in the first set, winning the tiebreak with the help of an ace and several other big serves. She was behind 2-0 and 3-1 in the second set before winning 6-4 against a visibly frustrated Azarenka.
"I'm really happy that I was able to come back in those two sets when I was down," Zvonareva said. "I think I shouldn't have started that slow, because I gave her an advantage right away. But at the same time, I was just trying to find my rhythm. I was trying to play aggressive. Of course when you're taking a little bit more risk, those mistakes will come."
Azarenka, the 10th-ranked player who was invited to the tournament because of the injury to Williams, chalked up her loss to several missed chances.
"Yes, definitely it was a close match. I had a lot of chances," he said. "Very unlucky at some moments ... But, you know, it's part of the game. You just have to deal with it. I think I played a good match. I'm still in the group. I still have chances to play further."
The match between two good friends lasted more than two hours and was the most entertaining of the tournament to date, with several rallies of 10 shots or more.
Azarenka, coming off a victory in the Kremlin Cup, got off to a strong start, helped by some erratic play by Zvonareva. But Zvonareva got going late in the first set, hitting several aces and backhand winners down the line. She leveled at 6-6 and then raced out to a 3-0 lead in the tiebreak.
The second set got off to a similar start as the opener, with Azarenka the aggressor. But up 3-1, Azarenka began to struggle, hitting several double-faults and shots that sailed long. Helped by a strong backhand and improving serve, Zvonareva took the lead at 5-4 and closed it out when Azarenka hit a shot long.
The fourth-ranked Clijsters was out with a foot injury and hadn't played since winning the U.S. Open in September, but she won the first eight points on her way to a comfortable win Wednesday.
Jankovic said after her opening match -- a loss to Zvonareva -- that she was ill and had collapsed in the locker room after complaining of dizziness. The Serb took a 2-1 lead in the second set, but Clijsters came back to make it 4-2 before closing out the match when Jankovic double-faulted.
Clijsters looked comfortable on the court, moving easily and pulling off several cross-court winners. She did show a bit of rust, committing 10 double-faults to go along with her seven aces.
"It wasn't my best tennis but still good enough to win in two pretty easy sets," Clijsters said. "The score was a lot easier than it was. But I felt I did a lot of good things. Also a few little things that I just need to tweak better by tomorrow. Just need to, yeah, just do a few little things better."