World number one Justine Henin [Images] qualified for the WTA Championships semi-finals with an easier-than-expected 6-2, 6-2 win over Serb Jelena Jankovic on Wednesday.

The Belgian's second straight sets win in the yellow group of the end-of-season tournament had put her on the brink of the last four and she qualified when American Serena Williams [Images] retired injured from her opening match against Anna Chakvetadze [Images].

The 20-year-old Russian was leading 6-4 after a topsy-turvy first set when Williams stopped with a problem in her left knee, putting her participation in the rest of the event in doubt.

The former world number one had looked short on mobility throughout, and was visibly suffering after having a lengthy medical break with the score at 5-4.

Williams was having a scan later on Wednesday but does not have to decide whether to continue in the championships until just before her next match. Marion Bartoli of France [Images] is on standby to step in if the American drops out.

In the red group, Serbia's Ana Ivanovic, who edged Svetlana Kuznetsova on Tuesday, moved a step closer to the last four beating Slovak Daniela Hantuchova [Images] 6-2 7-6.

After rattling through the first set, the world number four was forced to battle it out in the second as both players broke twice to go into a gripping tiebreak.

They traded match points and set points, before the pressure got to Hantuchova and she netted a forehand to lose 11-9 in the tiebreak after a two-hour, 12-minute contest.

Hantuchova's second straight-sets defeat means the world number nine has no chance of making it to the last four.

TOOK CONTROL

Henin, who beat Chakvetadze 6-1 7-6 on Tuesday, took control early on against Jankovic, breaking the world number three's serve three times in an error-strewn first set from both players.

The 22-year-old Serb, making her first appearance in the WTA Championships, struggled to get into her stride and double-faulted to lose serve at the start of the second set.

A marathon 10th game went Henin's way with a tight call on an ace, and she broke her opponent again in the next.

Although the Belgian lost her serve to go 3-1 in the second set, she held firm to inflict her seventh defeat of the season on Jankovic in one hour 28 minutes.

"I'm happy. I've been very aggressive, it was very intense and I played a good match. From the first point to the end I just took all my chances, and it was her first match so maybe I had a little more of an advantage," said Henin.

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