Istanbul - Petra Kvitova stayed on course for her biggest title since winning Wimbledon when she overcame Agnieszka Radwanska 7-6 (7/4), 6-3 to score her third win at the WTA Championships on Friday.
The Czech’s success carries her unbeaten momentum into the semi-finals and denied the Pole, who needed just one set to reach the semi-finals as well, by a frustratingly narrow margin.
Radwanska was within two points of achieving her goal at 5-2 and deuce in the first set, but Kvitova consistently found her flat-hitting best to pull back the deficit, looking a formidable attacking force as she did so.
"I just found the rhythm when I really needed it," she said.
Kvitova’s win also lifted her - temporarily at least - above Maria Sharapova into the world No 2 ranking.
Whether she holds on to that until the end of the season will also depend on the further progress of Victoria Azarenka who is also in the semi-finals.
Vera Zvonareva, who missed three match points against Radwanska on Thursday, qualified for the semi-finals instead of the Polish player and will face Azarenka in Saturday's semi-finals.
The other semi-final will see Kvitova take on Australia's Samantha Stosur.
Both Radwanska and Zvonareva scored one win and three sets each but the Russian had a slightly higher games won percentage.
Radwanska missed her best chance in the moments after she had advanced to 5-1 in the first set, with the advantage of a double break.
But then she played her first indifferent game of the match, losing her serve to love, and once Kvitova was allowed back into it she was a different, far more dangerous player.
In the eighth game, when Radwanska was at deuce and closest to reaching the semi-finals, she was allowed no chance in the next two rallies.
Kvitova launched a battering flat backhand down the line, setting up a short high return which she pummelled away with a drive volley. She followed it with an ace and a triumphant yelp.
Those yelps became more frequent as the match wore on, and although Radwanska used her intelligent, well-controlled game to break back bravely from 0-2 to 3-3 in the second set, the reprieve was temporary.
The match turned in the next game after Kvitova saved a break point by converting defence into attack and concluding the rally with another strident volley winner.
It seemed to break Radwanska’s considerable resolve.
After winning that game, after four deuces and 13 minutes, Kvitova broke serve to love for 5-3 and quickly closed out the match.
The fourth semi-finalist is Samantha Stosur, the US Open champion from Australia, who trampled over Li Na, the French Open champion from China, by 6-1, 6-0.
Stosur was delighted at how she turned her form around completely from the heavy beating she took against Azarenka two days ago, but Li was mystified at how heavy-legged she felt throughout.
Since her Roland Garros triumph the woman from Wuhan has won only seven matches.
She said she was prepared to consider taking on a new coach, a role which since 2006 has mostly held by her husband, Jiang Shan.
"I mean if I really have a nice coach, of course, I would take it," Li said. "But I have to see the chance. On this tour it is really not easy to find a good coach, a nice person."
Azarenka, already sure of finishing top of her group, lost her third and last round-robin match to France’s Marion Bartoli, a substitute for the injured Maria Sharapova who withdrew after two matches.
Bartoli’s 5-7, 6-4, 6-4 success may have been a bit-part role in a sideshow, but there was a significant consolation - prize money of $115 000 for one match.
Bartoli has now finished the 2011 season with 58 wins, second only to world No 1 Caroline Wozniacki, and played more matches than any other player - 82, two more than Wozniacki.
Results from fourth day of the season-ending WTA Championships on Friday:
Samantha Stosur (AUS) bt Li Na (CHN) 6-1, 6-0
Marion Bartoli (FRA) bt Victoria Azarenka (BLR) 5-7, 6-4, 6-4
Petra Kvitova (CZE) bt Agnieszka Radwanska (POL) 7-6 (7/4), 6-3