Hopes of two Australians featuring in the last-eight of the prestigious $US20 million ($A30m) event were ended on Saturday morning when Lucas Herbert played superbly, only to be out-duelled by Rory McIlroy in a high-quality last-16 clash.
In the afternoon session, Day held nothing back against local hero and reigning champion Scheffler, who had to win the last two holes to come from behind to beat J.T. Poston 1 up in his previous tie.
Day, winner of the event in his heyday in 2014 and 2016, made three birdies and an eagle in the first six holes for a 3-up lead, but as he did in the morning, Scheffler proved himself fully capable of a dynamic rally.
It began with a winning par at No.8, followed by a winning 29-foot birdie at the par-4 ninth to trail by one at the turn. The next two holes were tied with pars, before Scheffler levelled the match with a two-putt birdie at the par-5 12th.
Scheffler also birdied the par-4 13th, his two-putt par won No.14 and Day stood 2-down with two holes to play.
The Masters and Players champ sealed the deal at the short par-3 17th, stuffing a wedge to tap-in range for a conceded birdie.
Scheffler will face compatriot and good friend Sam Burns in the final four after the world No.13 beat Canadian Mackenzie Hughes 3&2.
Earlier, former world No.1 Day ended the adventures of US milestone man Matt Kuchar, winning 1 up after a tight contest.
It continued the 35-year-old Queenslander's stellar form at Austin Country Club as he followed up his group victories over Adam Svensson (4&2), Frenchman Victor Perez (2&1) and two-time major winner Collin Morikawa (4&3).
Bendigo's Herbert played some tremendous golf, matching world No.3 McIlroy shot for shot over much of the morning.
He finished up six-under-par for his round but the Northern Irishman proved stronger, holing out consistently well on the home stretch and birdieing the last three holes to win 2 up.
McIlroy then ended world No.6 Xander Schauffele's hopes, winning 1 up to set up a semi-final showdown with Cameron Young, who won the all-American tie against Kurt Kitayama 1 up.
