CHICAGO, 17 Sept 2015, (AFP) - Day launched his bid for a victory that could lift him to number one in the world in scintillating fashion, an eagle and nine birdies with one bogey putting him 10-under when the horn sounded to end play shortly after he teed off at his final hole.
With play then suspended for the night, Day was four strokes clear of his nearest rival, American Daniel Berger -- who was in the clubhouse on six-under 65 after holing out for an unlikely eagle at the par-four ninth.
"Being able to be aggressive, more aggressive with my drives off the tee boxes here has definitely paid off," Day said after his first-round heroics.
"I'm just trying to -- just rolling with it right now, and it's worked out."
American Brendon Todd was in the clubhouse on five-under-par 66, while Spieth was five-under through 17, Kevin Chappell five-under through 16 and Justin Thomas five-under through 13.
Day, however, was the undisputed star of what was quite a show.
"I'm walking through the locker room and people are going, you're playing off the ladies' tees, or you're playing a different golf course, or every time you stand up there and hit a drive it's downwind for you," Day said.
"I'd like to say all of those things are true, but they're not. It's good to see that the guys are recognizing that I'm playing good. Obviously it's a good round. I'm not thinking about it too much. I don't want to get too high or too low."
He opened with back-to-back birdies from within six feet on the 10th and 11th holes at Conway Farms Golf Club, Illinois before rolling in a 37-footer for birdie at 13.
He drove the par-four 15th and two-putted for another birdie before giving back a stroke with a three-putt bogey at 17.
He followed that miscue by picking up five strokes in his next four holes with a birdie at 18, a hole out from a fairway bunker for eagle at the first and birdies at the second and third.
He could only laugh when his approach from the bunker at the par-four first hit the green, veered right and rolled in.
Shortly thereafter, playing partner Spieth saw his seven-iron from the second tee land just short, bounce forward and roll into the cup.
The ace jump-started a round that saw Spieth even through his first 10 holes with two bogeys and two birdies.
The Masters and US Open champion -- left searching for answers after back-to-back missed cuts -- followed up with a chip-in birdie at the third and birdies at the fourth and seventh to reach five-under.
World number one Rory McIlroy, meanwhile, was three-under through 12 holes when play was halted with 52 players still on the course.
The first round was to resume Friday morning, with second-round tee times moved up because of a threat of more storms on Friday afternoon.
The three-way tussle for the world's top ranking is just an added spice to the penultimate tournament in the US PGA Tour's FedEx Cup playoffs -- in which a $10 million playoff bonus will be up for grabs in the Tour Championship.
Of this week's field of 70, only 30 will advance to the season finale, where all the remaining players have a theoretical shot at the bonus and the top five in the standings going in will be assured of capturing the big prize with a victory.