Leona Maguire battled back from four down to defeat South Korea's Jenny Shin to take control of Group 7 at the Bank of Hope LPGA Matchplay in Las Vegas.
Having defeated Linnea Strom 4 & 2 in her opening group game, Maguire made a poor start on Thursday, falling 4 down in her first eight holes.
Maguire, currently ranked 19th in the world, won the 9th, 11th and 14th to fire back into contention. The comeback was completed as she won 15 to level the contest, however, she immediately went 1 down after losing 16.
That teed up a dramatic finish, with Maguire winning the final two holes to finish the match 1 up.
On Friday, she faces Shin's compatriot Na-Rin An in her third group game.
The 64-player field is divided into 16 groups that are competing in round-robin play through Friday. The winner of each group will advance to single-elimination play.
Nine other players, including Celine Boutier of France, earned a second consecutive group-stage victory on Thursday.
However, no players are guaranteed a place in the knockout round, with all 16 groups to be decided when round-robin play concludes on Friday.
Boutier downed the United States' Sarah Schmelzel 2 and 1 to sit atop Group 3 at 2-0-0. Boutier will oppose South Africa's Paula Reto (1-0-1) on Friday,with the winner advancing to the weekend. A tie would see Boutier move on.
The day's final match resulted in the biggest upset, as Switzerland's Albane Valenzuela defeated top-seeded Lilia Vu of the United States 2 up.
Vu birdied the par-3 17th hole to cut her deficit to one hole, but she double-bogeyed 18 as Valenzuela made par to seal the win.
"Definitely very happy to walk away with a win," Valenzuela said. "I mean, I knew this was going to be a very tough match. Lilia is undoubtedly one of thebest players in the world and made me work for the points.
"She definitely left some putts out there, and I did as well, so we kind offought till the end, but just happy to come out with a win."
With additional material: Reuters