Doha, Qatar -- (Sports Network) - Serena Williams beat sister Venus in straight sets on Sunday to capture the title at the Sony Ericsson Championships.

The second-seeded Serena notched a 6-2, 7-6 (7-4) triumph for her third title of 2009 and the 35th of her brilliant career. Her only other wins this year came at the Australian Open and Wimbledon, but she will finish 2009 as the top-ranked women's player in the world.

"Playing Venus in the finals was tough," Serena said. "Even though she wasn't feeling good, she was getting every ball back. I can't tell you how frustrating that is! Playing her is never easy."

It was a perfect week for Serena, who was 3-0 in her round-robin group and beat Caroline Wozniacki in the semifinals. Wozniacki, the U.S. Open runner-up from Denmark, retired in the second set on Saturday.

Venus, the seventh seed and defending champion, battled a knee injury this week and finished second in the four-player round-robin group despite a record of 1-2. She then gutted out a three-set win over Jelena Jankovic in the semifinals.

A rested Serena was simply too much on Sunday, winning the first set with a pair of breaks. She didn't face a break point during the match and improved to 13-10 lifetime against her sister. That includes a 5-7, 6-4, 7-6 (7-4) win during round-robin play earlier this week and a straight-set victory in the Wimbledon final.

"I have to congratulate Serena for playing a great match today and having a great tournament," Venus said. "Last year I was able to win Wimbledon and the Sony Ericsson Championships, and it felt great. This year I came close to winning both, but it just didn't work out for me. I'll just continue to learn from everything and continue to enjoy what I do."

In addition, Serena improved to 8-3 against Venus all-time in title matches.

Serena also earned a first-prize of $1.55 million as an undefeated champion.