And all achieved despite a back problem which plagued him throughout the tournament, ignoring pleas by his support team to withdraw with the first major of the golf season just around the corner.
"I just want to win," Day told the official PGA Tour website.
"I wanted to win so bad that I felt with how I was playing, if I kept playing the way I was doing, I would be holding the trophy at the end of the week.
Day wrested top spot from Jordan Spieth Saturday when the Texan exited the match play event in Austin at the last-16 stage to Oosthuizen, while the Australian marched through to the semifinals.
It set up a titanic last-four clash with world No. 3 McIlroy, an acknowledged expert in these head-to-head encounters.
Day -- who won last week's Arnold Palmer Invitational at Bay Hill, helped by texts from his friend Tiger Woods, still sidelined with long-term back problems -- started strongly by rolling home a birdie on the opening hole to go one up.
But McIlroy, in fine form himself heading into the opening major at Augusta in 11 days' time, hit back with three birdies to lead one up through the turn.
Day immediately leveled with a birdie on the 10th and after McIlroy missed a chance to take the lead again on the 11th, he seized his chance.
Birdies on the 12th and 13th took him two up and despite McIlroy cutting the deficit on the 14th, he could make no further progress.
Queenslander Day closed out the match by holing a tricky par putt on the 18th to end any hope of a comeback by his opponent.
"It was a good quality match, but I missed a couple of opportunities by not birdying the 12th and 13th," admitted McIlroy.
South African Oosthuizen beat Rafa Cabrera Bello of Spain 4&3 in the other one-sided semifinal, but was no match for Day in the title match, falling three down by the turn and eventually succumbing 5&4.
Day, who won his first major by claiming the 2015 U.S. PGA Championship, will be looking to double up in the U.S. Masters and may well head there at the summit of the global game.
Spieth, winner of two majors last season, had held top spot for all of 2016 and can still regain it with a strong performance in next week's Houston Open.
Cabrera Bello won the battle for third spot 3&2 over McIlroy, who will be turning his attentions to claiming the only major to elude him on Thursday week at Augusta National.
