Kim Clijsters overpowered Wimbledon champion Venus Williams 7-5 6-2 to win the Bank of the West Classic title.
Williams, playing for the fifth consecutive week, was well off the pace of her opponent as she made a series of unforced errors.
Clijsters registered Williams' fatigue and ensured she did the simple things well, engaging in long rallies against the number two seed.
"It's always good to come up with such a good victory," said Clijsters.
"She's a great player and I knew I'd have to play my best to beat her. I stayed patient and did a lot of things right.
"In the past, I think I tried to do too many things against her and I got out of my normal rhythm.
"I was mixing and defending well. I think I put her off a few times."
Williams admitted her punishing schedule had taken its toll on her recent performances.
"I tried but didn't have enough energy," said Williams.
"It's very challenging to play as much as I've been. Not only physically, but also mentally. I need to be mentally fresh to compete and if I'm not, it's hard to stay in there."
The victory will help Clijsters to return to the top 10 for the first time since October 2004, as she continues her remarkable comeback from wrist surgery.
Williams will now take a week off before travelling to Stockholm for the WTA Nordea Nordic Light Open.