Grace and Oosthuizen were undefeated together in five Presidents Cup matches before the emerging star pairing of Fowler and Thomas schooled them 3 & 2 on in four-ball on Friday.
In a tense match which went to the 17th hole, South African pairing Louis Oosthuizen and Branden Grace took the first win of the day for the global team over Koepka and Daniel Berger, winning 3-and-1.
United States 8, global 2.
The International team can be backed at 25/1 for a remarkable win and the tie at 15-15 is available at 20/1.
The perfectly judged effort from the 24-year-old was met by rapturous applause from the watching galleries and Thomas was keen to raise the noise levels even further following his timely birdie.
Hoffman and Chappell are Presidents Cup rookies and promptly earned a point. Live coverage continues on Saturday behind the red button from 1.00pm and uninterrupted from 5.00pm.
Dustin Johnson and Matt Kuchar pushed the reeling Internationals into a deeper hole with a 4&3 decision over Canada's Adam Hadwin and Australian Adam Scott. "We've got a ways to go yet and we'll keep telling them that".
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The International team, who won at Royal Melbourne in 1998 and secured a tie at Fancourt in 2003 but have lost the other nine matches, look to be heading for a heavy defeat after making the United States work hard for their win in 2015.
Hoffman and Chappell were three up after the first four holes, before birdies at eight and nine saw them take a five hole lead into the back nine.
The Internationals troubled the scoreboard when Matsuyama and Adam Hadwin secured a half point by squaring their match with the powerful Reed/Spieth combination, but that's where it ended for the International side who will return to their Manhattan hotel in a state of shock, their tails wedged firmly between their legs and with the sound of overjoyed American crowds ringing in their ears.
This rematch from Friday's four-ball session was the most competitive match of the morning. They both missed 8-foot putts to win holes early in the match. To get a half out of it was probably disappointing, but also pleasing.
The Internationals have again been troubled by the USA depth, with India's Anirban Lahiri having a nightmare in his match alongside Charl Schwartzel.
On the 14th hole, Thomas chipped in from a greenside bunker for birdie to maintain his team's 2-up lead.
Day and compatriot Marc Leishman suffered a heartbreaking loss to Phil Mickelson and Kevin Kisner on Friday, leading from the first through the 14th holes only for Mickelson to grab a 1-up win for the United States at the 18th.
In amongst that he virtually left Schwartzel one-out against the Americans with a string of bad shots. "The South Africans built a 2 UP lead thru six holes, including circles at holes 2 and 6, with the Americans winning just the fourth with birdie-4, but they lost holes 7-9, beginning with a 3-putt bogey at 7 from 59'9" and a sloppy double bogey-6 at 8.
Oosthuizen will team with Day against Spieth and Reed in the first afternoon match.