Serena and Venus Williams will meet for the 23rd time on the professional tour in the final of the Sony Ericsson Championships on Sunday.

The sisters won their semi-finals in contrasting fashions today, with Venus battling to a 5-7 6-3 6-4 victory over Jelena Jankovic while Serena was leading Caroline Wozniacki 6-4 0-1 when the Dane retired.

Serena is guaranteed to be world number one at the end of the season and she made it four wins out of four at the season-ending tournament in Doha.

Wozniacki has been suffering from a leg injury all week and, after giving the second seed a real battle in the first set, called it a day having broken in the opening game of the second.

Williams paid tribute to the battling qualities of her teenage opponent before turning her attentions to the family affair.

The pair have already met this week, with Serena saving a match point before defeating Venus in their group match.

That took the 28-year-old's advantage in their head-to-head record to 12-10, with Serena bidding for a fourth victory in a row after also triumphing in Miami and in July's Wimbledon final.

She said of Wozniacki: "Just to come out today and try to put up a fight for the crowd, it's so good to see a face like hers on the Sony Ericsson WTA Tour. She's an incredible fighter.

"I'm just taking it one day at a time. I've never done so well in the round robin of the Sony Ericsson Championships, so I'm really excited.

"I came into this week hoping to do my best and here I am. I'm literally giving everything I have. I think I'm going to be wiped out by the end of this," added Serena, who won the Tour Championships for the only time in 2001.

Wozniacki put a brave face on her withdrawal and reflected on a season that has seen her emerge as the leading challenger to the established faces.

"It was just too much for me to keep playing," said the 19-year-old on www.wtatour.com.

"I could either smile or cry, so I chose to smile.

"I had a great year. I did all I could. I used all the strength I had left. Today I just didn't have any left. I've learned that I'm a huge fighter and impossible is nothing."

Defending champion Venus has certainly been giving the crowd value for money and went the distance for a fourth successive match as she battled past Jankovic.

"The crowd really helped me through it," said the 29-year-old. "It was great.

"In the first set I couldn't feel the ball. Coming back from not being able to get a ball in to beat such a consistent player like Jelena is great. It's my last tournament of the year so I'm giving 1,000%."