Rahm, who last week was the only amateur to make the cut at the US Open, fired seven birdies at the event which benefits Woods's charitable foundation to lead Venezuela's Jhonattan Vegas by a stroke.
"I was really comfortable off the tee with the driver, hitting it good and hitting it straight, so that allowed me to be a little more aggressive and it paid off," Rahm said.
The start of the first round at Congressional Country Club in Bethesda, Maryland, was delayed by two hours due to thunderstorms in the area, but it did little to unsettle Rahm.
The 21-year-old, who started his round at the 10th, posted three birdies over his first nine holes before picking up another four on the way home.
