The top seed Serena Williams overcame fellow American CoCo Vandeweghe 7-5 6-3 to retain the Stanford Classic title.
Just eight days after winning her fifth Wimbledon single title, Williams fought off a set point in the first set before overcoming big-serving Vandeweghe, who had reached her first WTA final.
The 30-year-old Williams used her experience and know-how to race past the 20-year-old, nailing 21 winners.
Vandeweghe struck 20 winners and committed 25 unforced errors.
Williams won her 43rd career title, tying her with her older sister Venus for the most titles among active players on the WTA tour.
The world number four has now won 28 out of her last 29 matches.