Both Williams sisters went into the final with strapping on their left legs
Serena Williams beat her sister and defending champion Venus Williams to win the Sony Ericsson Championships.
The younger sister won a low-key final 6-2 7-6 (7-4) as both players struggled for fitness in Doha.
Venus, 29, had heavy strapping on her left knee, while Serena, 28, had similar taping around her left thigh.
But Serena, who will end the year as world number one, took charge of the season finale from the outset and came through in one hour 25 minutes.
"It's nice to finally win a tournament other than a Grand Slam," said the 11-times major champion who last won a title outside the big four in April 2008.
"We definitely weren't physically 100% out there today but it just shows you how much we try. Even in the second set, how hard we just both fought, especially Venus, she was getting every ball back.
You have to show up and play no matter what
"I've seen Venus return better, and I served well. I think I really did serve well."
Venus, who came back from a set down to beat Jelena Jankovic in the semi-finals on Saturday, said: "I've been playing for a long time. I'm not going to be the one to say, 'Oh, I played forever yesterday.'
"You have to show up and play no matter what. That really has nothing to do with it.
"I just made a slow start and although I served better in the second set I didn't serve as well as I wanted to."
It was the second time in four days that the Williams sisters had played each other, but the final came up well short of Serena's three-set win in the Maroon Group on Wednesday.
WTA Tour chief executive Stacey Allaster said afterwards that a new scheduling system with fewer mandatory tournaments saw player withdrawals go down by more than one third in 2009.
But the showpiece tournament in Doha was beset by injury problems throughout, with world number one Dinara Safina pulling out after just two games of her opening match, and her replacement Vera Zvonareva then withdrawing after her only tie.
Victoria Azarenka retired due to cramp in her final group encounter, while Caroline Wozniacki was in tears at one point during her last round-robin match due to cramp and could not complete her semi-final against Serena Williams.
So it was appropriate that the two finalists emerged onto court looking like they had come straight from the physio room, each with heavy strapping on their left legs.
Serena could still crank up the power, however, and broke serve in game three, while her elder sister was unable to call on her usual athleticism around the baseline.
Despite repeatedly wincing in pain, Serena rolled through the first set in half an hour and looked on course for a swift victory.
Venus was not about to give up her title lightly and raised her game considerably in a much better second set, but Serena was imperious on serve, dropping just two points as she forced a tie-break.
The 2001 champion appeared to be wobbling when she was pegged back from 5-1 up to 5-4, but an ace and a thumping forehand closed out her 13th win in 23 matches against her sister.
Victory in Doha completes a superb year for Serena that has included wins at the Australian Open and Wimbledon, and reclaiming the world number one spot.
There remains one more obstacle to overcome as the WTA will decide this month what action to take over her outburst during the US Open semi-final against Kim Clijsters, with a Grand Slam ban of some sort a possibility.