Scott, Min Woo Lee and Lucas Herbert all posted tense 1-up victories, while Day charged to a 4&2 win at the $US20 million event over Canadian Adam Svensson.
Cam Davis was the only Australian to fall at Austin Country Club on Wednesday, succumbing 4&3 to American sixth seed Xander Schauffele.
Scott was two down through 13 holes before rallying with birdies on 14, 16 and 18 to snatch victory from the jaws of defeat against Ireland’s Seamus Power.
Scott iced the win with a brilliant 26-foot birdie putt at the last.
It was a great start for the Australian as the 2013 Masters champion bids to progress to the knockout stages for the second straight year, two weeks before he celebrates the 10th anniversary of his landmark win at Augusta National.
Jason Day had the biggest margin of victory for the Aussie quad on day one. PHOTO: Tom Pennington/Getty Images.
Scott’s remaining Group 13 matches are against Sam Burns and Adam Hadwin.
Day was 1-down through 10 holes before rattling in six successive birdies in one of the rounds of the afternoon.
The two-time Match Play champion also faces Collin Morikawa and Victor Perez in Group 9.
Lee and Herbert both made winning starts earlier in the morning with thrilling but contrasting victories.
After leading for much of the morning, Lee had to come from behind to snatch a 1-up win over Sahith Theegala
A double bogey from Lee on the 14th hole, followed by a birdie for his opponent on 16 allowed Theegala to go 1-up with two to play.
But Lee pulled the match out of the fire with birdies on 17 and 18 ahead of further group matches against JJ Spaun and reigning U.S. Open champion Matt Fitzpatrick.
RIGHT: Chasing a Masters invite, Lucas Hebert got off to a winning start in Texas. PHOTO: Harry How/Getty Images.
There is much at stake this week for the 24-year-old Perth ace.
As well as needing a decent showing to seal a spot at next month’s Masters, Lee will earn Special Temporary Membership for the remainder of the PGA Tour season if he can top Group 11 – or finish in a two-way tie for 17th with two pool wins.
Ranked outside of the top-50 cut-off mark for Augusta National, World No.56 Herbert probably needs to progress to the quarter-finals or further this week to make it back to the Masters.
He’s at least on track after hanging on for his own 1-up win over Russell Henley.
Herbert powered to 5-up with five birdies on the front nine only for Henley to hit back on the inward nine to almost steal it.
Herbert’s next two matches in Group 14 are against Tyrrell Hatton and Ben Griffin.
After losing to Schauffele, Davis will likely need to beat both Tom Hoge and Aaron Wise to advance to the knockout stages from Group 6.
World No.1 and defending champion Scottie Scheffler, second seed Jon Rahm, third seed Rory McIlroy and Kiwi Ryan Fox were also among the day-one winners.
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