Williams sisters through to Doha Championships semi-finals

REUTERS - Serbia's Jelena Jankovic crushed leg-weary Caroline Wozniacki to reach the semi-finals of the WTA Championships while American holder Venus Williams reached the last four on Friday without striking a ball.

On a day when calculators were needed to work out the various permutations in the two groups of four, Wozniacki deservedly survived for the weekend.

The Dane progressed after Belarussian Victoria Azarenka's hopes ended in tears when she retired injured in the third set of her match against stand-in Agnieszka Radwanska of Poland.

In Saturday's semi-finals, world number one Serena Williams, a class apart so far this week, will face White Group runner-up Wozniacki while sister Venus, who is through despite two defeats in three matches in the Maroon Group, plays Jankovic.

Venus crept in on a sets countback after already eliminated Svetlana Kuznetsova beat fellow Russian Elena Dementieva 6-3 6-2 to leave three players tied on one win each.

Feisty Dane

The feisty Wozniacki, a surprise U.S. Open finalist last month, has gone through the full range of emotions on her first appearance at the WTA's showpiece finale.

After spending a total of almost six hours on court in winning her first two round-robin matches against Azarenka and first alternate Vera Zvonareva, she crumbled against Jankovic, losing 6-2 6-2 as she battled fatigue and an abdominal strain.

"I didn't have anything left. I couldn't play like I wanted to. I had some pain in my stomach muscle so I couldn't serve," the 19-year-old, who the day before was poleaxed with cramp against Zvonareva, told reporters.

Quite what shape she will be in on Saturday against Serena is a major question.

"It's still the hamstring. I also have my stomach iced," said the world number four. "My body is just tired now. I was hoping for a couple more matches here but after this tournament I'm ready for a vacation."

Needing Radwanska to beat Azarenka in the day's final match, Wozniacki looked resigned to her fate as she watched on from the stands when the Belarussian went 6-2 5-2 ahead.

However, Azarenka began to struggle with back and thigh injuries and Radwanska reeled off five games in a row to win the second set. A hobbling and tearful Azarenka needed regular treatment in the decider before quitting at 4-1 down.

Tidy sum

World number four Wozniacki said she deserved to reach the semi-finals but admitted it was tough hoping Azarenka, her closest friend on the Tour, would lose.

"Usually, I wouldn't cheer for any of them because they're both my friends. But now, in this situation, I think it's normal that I cheer for Radwanska," she said.

For her small but significant contribution to the tournament Radwanska pocketed the tidy sum of $200,000.

Jankovic, the last player to qualify for the lucrative tournament, has come a long way in three days.

On Tuesday she looked ready to head for the airport after a crushing defeat at the hands of Azarenka but the cards have fallen in her favour.

She was gifted victory in her second match when Dinara Safina retired after two games and on Friday Wozniacki, who she had beaten twice before, was clearly in no shape for a fight on another day of oppressive heat and humidity.

After sharing the first four games, Jankovic, who had spent 95 minutes on court compared to Wozniacki's energy-sapping total of 348, won the next eight on her way to topping the group.

"I knew that I had to win in order to go to the semi-finals, and if I lost this match I was out of the competition," Jankovic told reporters. "So it was a lot of pressure.

"I didn't know what to expect from Caroline, with all of the things that have happened in the last two days. I just came to really focus on my tennis. I did a good job."

With Wozniacki offering up cheap points all over the place, the biggest problem for Jankovic was holding on to her racket in the sweaty conditions that had her regularly wringing out her hair. "We are sweating a lot. It's very humid," she said.