DOHA: Caroline Wozniacki joined the WTA Championships casualty list when her hopes were shattered by a crippling stomach strain, but the bubbly blonde Dane showed she has plenty of guts for a fight.

The 19-year-old started 2009 at 12 in the world, but will finish at four while having also made the US Open semi-finals.

Furthermore, her resolve was tested when she was caught up in a WTA investigation after her controversial, injury-forced withdrawal from a low-key event in Luxembourg.

On her debut at the money-spinning season-ending tournament, she captured hearts with her tear-stained, cramp-wracked victory over Vera Zvonareva which left her barely able to walk.

Her on-court collapse and her struggles to finish off the last two points will be defining images of this 4.55 million dollar showpiece.

Against Serena Williams in the semi-finals on Saturday, with her left thigh heavily bandaged, her victory hopes ended at 4-6, 0-1 when her side strain became one burden too many.

"I could either smile or I could cry, and I chose to smile. There's nothing else you can do. I had a great year. I did all I could. I used all my strength I had left, but today, I just didn't have any more left," said Wozniacki.

"I've had a great tournament. It doesn't matter that I couldn't really finish today."

Wozniacki, one of the game's brightest talents and with a love of sport coursing through her veins thanks to her football-playing father and coach Piotr, will face sterner examinations next year.

As well as the Williams sisters, there will be the return of former number ones Justine Henin and Kim Clijsters to contend with.

But Wozniacki believes she has learnt valuable lessons over the last week in the hot and humid Gulf.

"I learned that I'm a huge fighter and impossible is nothing," she said.

"I just need to keep improving. Next time I step on the court, I will try to play even better than I did here."