Northern Ireland's Olivia Mehaffey lies tied for 11th after the second round of the inaugural Augusta National Women's Amateur Championship as a level-par 72 left her one over for the tournament.
Fassi, Kupcho's good friend and the 2018 ANNIKA Award victor as the top female collegiate golfer, birdied two of her final three holes to record a second consecutive round of two-under 70 for a four-under-par 140 total through two rounds.
Kupcho patted her friend's shoulder when Fassi answered with a shot that rolled back to a foot for birdie on the next hole. The iconic par-3 16th changed everything.
Kupcho, who started on No. 10, was 3 under for the round through 10 holes and poised to stretch it even more when she missed a short birdie putt on the next hole. Fassi, meanwhile, left herself an incredibly hard second shot and three-putted the tricky green to bogey the hole.
"All the emotions, once you get up there, the people are cheering and clapping, all those moments, they come back and you get goosebumps", she said.
"Lost my confidence a little bit", she said.
In the playoff, nine players safely negotiated the par-4 10th hole with pars, leaving Ainhoa Olarra of Spain and Alessia Nobilio of Italy fighting for the final place.
Fassi answered with a 10-foot birdie putt to regain the lead, and Kupcho chose to aim her 3-hybrid to the bunker right of the green on the par-5 15th. Sorenstam and her daughter walked with the final group.
The 12th hole is shown at "Amen Corner", where golfers turn the corner on the 11th, 12th and 13th holes, at the Masters golf tournament at the Augusta National Golf Club in Augusta, Georgia, April 6, 2018. She was hitting great shots.
"I did all I could, she played a great game she finished really strong and I am really proud of her and I know we are going to be battling for wins in the future", said Fassi. The final three hours of play will air on NBC sports starting at noon.
While the sight of women battling for a trophy was new for Augusta National, so were the sounds as the galleries offered encouragement with shouts of "Atta girl" and "Way to go girls".
Kupcho hopes the rest of it will look familiar. "So I think we'll make a lot of birdies, and it will be pretty fun to watch us".
However, she added: "I've only been in February so the conditions were really different to what I'm going to get the week before the Masters".