Zhang stuns Sagstrom to win Founders Cup
The 20-year-old from California rattled off four birdies in her final five holes to card a six-under par 66 to finish on 24-under, two shots clear of Sweden's Sagstrom.
In cool, overcast conditions at the Upper Montclair Country Club in Clifton, overnight leader Sagstrom had looked to be cruising to victory after rolling in her fifth birdie of the day on the 12th to go three shots clear.
Sagstrom retained that hefty three-shot cushion after parring the 13th along with Zhang, who gave no clue of the pyrotechnics to follow.
Zhang pulled a shot back with a birdie on the par-five 14th, and then got to within one after a superb tee-shot set up a birdie chance on the par-three 15th which she converted.
With the pressure building, Sagstrom then cracked on the 16th, under-hitting a chip onto the green which left her a long par putt that she missed to card a bogey five.
That left Zhang as co-leader and the youngster duly seized her chance.
Another dazzling tee-shot on the par-three 17th left her a three-foot putt for birdie which she rolled in to move one shot clear, and then she sealed victory on 18 with a nerveless birdie putt from 10 feet.
"I'm still shaking right now, but I think I never gave up," Zhang said afterwards.
"I always knew that I had something in me to just grind it out and enjoy the time against an extremely solid player.
"She was basically hitting every single shot next to the hole and making putts. So I had to prepare myself for the toughest challenge. But it's crazy that this happened."
Sagstrom, 31, meanwhile was left fighting back tears after seeing what would have been only her second LPGA Tour victory plucked from her grasp in agonizing fashion.
"It's golf; she played fantastic. I was playing well, and hitting the ball good enough I thought. I hit decent shots but it wasn't meant to be," Sagstrom said after her closing 69.
Pure nerves
Sagstrom, who led by one shot after Saturday's third round, admitted she had been affected by nerves early on Sunday, when she bogeyed the first.
"It was just pure nerves," she said. "I played unbelievable golf this week, so I have to look at the positives. It sucks, and it hurts a bit. But my time will come."
Sagstrom and Zhang had been effectively playing a match-play duel after racing clear of the field in the third round. Zhang had started the day 10 shots clear of her nearest challenger.
That big advantage continued through the completion of Sunday's final round, with Australia's Gabriela Ruffels finishing third on nine-under, 15 shots off the lead.
Chinese duo Yin Ruoning and Lin Xiyi finished tied for fourth alongside Taiwan's Chien Pei-yun on eight-under.
World number one Nelly Korda's bid for a record sixth straight title ended with a one-over par 73 that left her tied for seventh place with four other players.