Serena Williams claimed a fourth successive win over sister Venus to triumph at the Sony Ericsson Championships in Doha.

Serena, who was guaranteed to end the season as world number one, eased to a 6-2 7-6 (7/4) victory.

She wasted little time taking the first set but had to dig deeper in the second before winning the tie-break to go 13-10 up on defending champion Venus in professional meetings between the world-beating American siblings.

Venus had a battle on her hands to make the final, coming from a set down to sneak past Jelena Jankovic - when Serena had a much easier time, thanks to Caroline Wozniacki's mid-match retirement because of a leg injury.

Serena pasted a succession of service returns and rarely needed to get out of a canter - with long rallies a rare occurrence - as she closed out the first set with ease.

She had an early break and comfortably built on that advantage thanks to some typically crunching serves when she needed them - despite appearing in pain with a troublesome injury that required significant strapping on her left leg.

Venus, by contrast, rarely appeared capable of testing her sister and lost the final game of the first set to love - on Serena's serve.

Although Venus held serve throughout the second set, it was not until 40-0 down in the sixth game that she won her first point against serve since midway through the first.

And Serena continued to hold her serve with ease.

Yet Venus' persistence meant the break proved elusive until the balance of power was accurately reflected in a tie-break that saw Serena end the match with a rasping cross-court winner.

Serena insisted afterwards her near faultless campaign had far exceeded her expectations as she collected a second Tour Championships title.

"I didn't expect to come here and win," she told British Eurosport.

"My practice was not going very well at home - but I kept fighting.

"Playing Venus in the final was really tough. Even though she was not feeling all that great, she was getting every ball back - and it was really frustrating."