World number one Justine Henin will face a combination of ambitious newcomers and former winners when she defends her WTA Championships title in Madrid on Tuesday.

The 25-year-old Belgian has been in superlative form this season, claiming a career-high nine Tour trophies including the Grand Slam titles at the US Open and Roland Garros.

She heads into the $3-million tournament in Madrid on a 20-match winning streak and although she is aiming to end the season on a high she is not under the same pressure as last year when she was competing for the year-end No 1 spot.

"I would prefer 10 titles to nine this year for sure but it's probably for me the most difficult tournament of the year," Henin told Reuters last week.  "It's the best eight players so it's going to be a tough one. I know I'm going to be number one now so it's just going to be a challenge to win it one more time."

Among the Belgian's opponents is a rejuvenated Serena Williams who has qualified for the event for the seventh time after powering back into the elite. Another former winner, Maria Sharapova, made the cut after Serena's sister Venus was forced to withdraw on medical advice.