Serena Williams won her second career WTA Championships title on Sunday with a commanding 6-2 7-6 (7/4) victory over sister Venus, a triumph which earned her a cool $US1.55 million ($A1.69 million).

Serena, who will finish the season as world No.1, also won the trophy in 2001, making her just the second player after Justine Henin in 2007 to capture two Grand Slam titles as well as the WTA Championship in the same year.

Serena won all five matches at this event, including a group stage win over defending champion Venus, and such was her dominance in Sunday's final that she gave up just seven points on serve.

The 85-minute romp was her 13th victory in 23 meetings with her sister, a family rivalry stretching back to 1998.

"It feels fantastic. I definitely wasn't expecting to win when I came here because I hadn't been practising very well," said Serena.

"I kept fighting against a really tough player but Venus kept hitting the ball back. That's never easy and it's really frustrating."

Venus, who also lost her Wimbledon title to Serena this year, said she did all she could in the final.

"I gave 100 per cent. It's the end of the season. It's been a good week and I have no complaints. Serena played a great match. She didn't make many errors," said Venus.

With the last of her 41 career titles coming nine months ago, it will be little consolation for the 29-year-old that she will end 2009 at six in the world, up from seven.

In a symbol of this injury-hit, desert money-spinner, which had seen four players end their campaigns prematurely through various aches and pains, both sisters arrived on court nursing knocks.

Serena had her left thigh heavily-strapped while Venus had protection on her left knee which was treated several times during her semi-final win over Jelena Jankovic.

But Serena was the fresher of the two in the first set, a legacy of having spent three and a half hours less on court than her sister in reaching this stage.

Breaks in the third and seventh games gave the new world number one the opening set.

Serve dominated the second set before Serena stretched to a 5-2 lead in the tie-breaker. An ace set up match point and the title was secured with a crunching forehand winner.

Victory - the 35th title of her career - added to Serena's capture of the Australian Open and Wimbledon titles in 2009.

The $US4.55 million ($A4.97 million) WTA Championships, intended to showcase the top eight players in the world, were hit by a series of injuries.

Dinara Safina saw her world No.1 spot swept away by a flood of tears which accompanied her withdrawal because of a back injury after just two games and 12 minutes on court against Jankovic.

Caroline Wozniacki, who overcame crippling cramps to beat Vera Zvonareva, was forced to quit her semi-final against Serena to avoid further damage to her thigh and side.

The Danish teenager played 90 matches in 2009.

Victoria Azarenka was also unable to complete a match, handing victory to Agniezska Radwanska, who had herself started the week as the second alternate but replaced Zvonareva who had moved into the draw instead of Safina.