The putt gave the Itawamba AHS senior from Fulton a sudden-death victory against Ginger Howard of Bradenton, Fla., in match play on the Country Club of North Carolina course in Pinehurst, N.C.
Howard led 2 up with two holes to play when McDonald tied the match with back-back-birdies on 17 and 18. She sank a 15-foot putt on 17 and 6-footer to force the playoff.
"I was just trying to keep my head on straight and not get down," McDonald said. "I executed my shots. Without God, I wouldn't have made it today.
"It was a very long day ... a hot, humid day."
Thursday morning, McDonald defeated Puerto Rico's Kyle Roig 1 up in the round of 32.
In today's elite eight of match play, McDonald plays against the world's top-rated junior golfer, Victoria Tanco of Argentina. Tanco defeated Kristen Park of Buena Vista, Calif., 5-3 Thursday to reach the quarterfinals.
"She's a really great golfer, but I played two really great golfers today," McDonald said. "I've got to keep my same game plan. I'll have a good mindset and not let what she does affect me.
"The only control I have is over what I do."
McDonald's caddie for this tournament is her high school coach, Michael Nanney.
"He told me to just hit the center of the greens and make good putts," she said.
The winner of the McDonald-Tanco match will advance to today's afternoon semifinals. The finals are scheduled for Saturday.
McDonald posted a two-day, stroke-play score of 6-over-par 150 to qualify for match play. She defeated Kaitlin Park of Tustin Ranch, Calif., 2 and 1 in Wednesday's first round.