First alternate Angelique Kerber might be able to play at the season-ending WTA Finals in Singapore if one of the three injured players is not able to compete.

World No. 1 Serena Williams might not be able to participate because of a left knee injury that caused her to withdraw her quarterfinals match against Australian Samantha Stosur at the recent China Open in Beijing.

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''It throbs just sitting, standing,'' she said. ''I felt it mostly serving because I'm landing on my left knee. That was really a killer for me.''

Canadian star Eugenie Bouchard had her left thigh taped during her opening matchup win against Austrian wild card Patricia Mayr-Achleitner at the Generali Ladies Linz and said that the injury worsened overnight.

"I woke up this morning and it was much, much worse," she said. "It's hard for me to keep playing and keep making it worse."

She also withdrew from this week's Luxembourg Open.

The 2008 Australian Open champion Ana Ivanovic also pulled out at Linz after her first round match with a hip injury.

Kerber, 28, is actually in 10^th place of the eight-woman short list with China's Li Na in 9^th but Na recently retired from tennis which leaves the German as first alternate and if one of the three players mentioned withdraws from the season-ending event, Kerber will be playing in Singapore.

This would not be the first time for Kerber to be an alternate, in fact, she was also an alternate last year and was able to play after Maria Sharapova pulled out due to a right shoulder injury.

The year-ending tournament will also include a second alternate and for that, five players are still in contention namely Ekaterina Makarova, Dominika Cibulkova, Sara Errani, Jelana Jankovic and Flavia Pennetta.

Of the five mentioned, only three will participate in this week's Kremlin Cup, Makarova, Cibulkova and Pennetta which means Errani and Jankovic is already out of contention.

TagsAngelique Kerber, serena williams, Ana Ivanovic, li na