Australia's former world No.1 continued his stellar form to shut out Morikawa 4&3 to top his group and surge into the last 16 with a perfect 3-0 record at Austin Country Club.
After taking down Canadian Adam Svensson (4&2) and Frenchman Victor Perez (2&1) in his opening two matches, Day only needed a tie on Friday to advance.
But he throttled Morikawa from the get-go as a weather warning threatened to interrupt proceedings.
Day birdied three of his first four holes to go two up, doubled his lead with a sixth birdie in 12 holes and only showed any mercy when he three-putted the 13th to allow Morikawa to square the hole with a par.
But the let-up proved merely a stay of execution for the American ninth seed as Day secured at least the half-point he needed when both players recorded pars on the 14th hole to go dormie four.
Another pair of pars on 15 sealed it for Day, who remains very much in the hunt for a third triumph - and the $US3.5 million ($A5.3 million) winner's cheque - at the final edition of the popular event.
"I had a lot of good putts roll in," Day said.
"It would have been very difficult to beat Collin today if he'd holed some putts. He just missed his fair share of putts today and fortunately I made enough for both of us."
The tournament winner in 2014 and 2016, Day faces milestone man Matt Kuchar next for a place in the quarter-finals.
Kuchar earlier routed Si Woo Kim 7&6 to top his group after equalling Tiger Woods' record number of wins at the event.
The only three-time Match Play winner, Woods boasts a 36-12 record at the tournament. Kuchar is now 36-12, plus four ties, and can remove his legendary countryman from a page in golf's history books by eliminating Day on Saturday.
"Matching Tiger's record shows just how difficult he is to beat," Day said.
"He's a very patient guy. Doesn't get too ahead of himself or he doesn't get really down as well.
"Hits it very straight. If he has a good day on the greens, he's going to be difficult to beat. He's got a great short game.
"I just have to play to my own strengths and, if I can do that and put a little bit of pressure on him, great. Hopefully that makes a difference."
Finally free of the back problems that have dogged his career in recent years, Day looks a man on a mission as the former PGA Championship winner hunts down a first tour victory since 2018.
While the 35-year-old marches on, fellow Australian former world No.1 Adam Scott (1-2) was eliminated with a 3&2 loss to Canadian Adam Hadwin (1-2).
American Sam Burns (3-0) topped that group unbeaten to progress after seeing off Ireland's Seamus Power 2&1.
American Max Homa was earlier the first player to reach the last 16, despite not hitting a single shot.
Homa was due to face former Masters champion Hideki Matsuyama in the final round of group matches, only for the Japanese to withdraw before play got under way with a neck injury.
That meant Homa was credited with his third win of the week and advanced to the knockout stages for the first time.
