Hannah O'Sullivan with the Robert Cox Cup (USGA photo)

Brooks, who will attend Wake Forest in the fall of 2016, was 3-up through 15 holes before O'Sullivan won holes 16 and 18 to reach the halfway point with a 1-up lead. But she won only three holes during the rest of the match, with O'Sullivan pulling away on the 15th hole to go dormie-three.

“When we came back out (from lunch), she definitely played 100 percent and she definitely brought it on,” Brooks told the USGA. “I struggled a little bit, but I still played well, and she just played better, and she made more putts, and in match play that's what it always comes down to.”

O'Sullivan rolled in a 3-foot par putt on the 16th hole, completing a quest she almost conquered a year earlier when she reached the semifinals at Nassau Country Club in New York.

“I can't really describe how good it feels,” O'Sullivan told the USGA, who also earned co-runner-up honors at the 2015 U.S. Women’s Amateur Four-Ball Championship. “It was just such a tough match today, and to be able to come out on top, it's just incredible. I just feel so blessed.”

O'Sullivan won a professional tournament earlier this year, becoming the youngest player to ever win on the Symetra Tour, while Brooks had victories at the Women's South Atlantic Amateur and Women's Southern Amateur.

Both O’Sullivan and Brooks receive exemptions into the 2016 U.S. Women’s Open Championship, which will be held July 7-10 at CordeValle in San Jose, Calif., just 30 minutes from where O’Sullivan grew up in suburban San Francisco.

O’Sullivan also receives a 10-year exemption into the U.S. Women’s Amateur and one-year exemptions into the Ricoh Women’s British Open Championship, ANA Inspiration, and Evian Championship. Brooks receives a three-year Women’s Amateur exemption.