Day won the 2015 US PGA Championship and finished a shot behind 2016 victor Jimmy Walker.
McIlroy remains one of the brightest talents in golf, but he has also failed to win a major since he claimed two in 2014.
Winless this year and having battled back and rib injuries, McIlroy suggested his 2017 could be over, but the Northern Irishman is unsure what his immediate future holds.
Thomas struggled off the tee, hitting just six of 14 fairways (though some of those were just off the short grass) and his iron play, despite 14 greens hit, wasn't as crisp or controlled as he knows it needs to be if he's going to hoist the Wanamaker Trophy on Sunday. "You might not see me until next year".
"But right now it's a tough one because I go out there and shoot decent scores, but when I come off the course I feel my left rhomboid going into spasm, and the inside of my left arm goes numb".
When Spieth competes in the PGA Championship next year in St. Louis, he will be older than Woods was when he earned the career Grand Slam at 24 years, 6 months old. April is a long way away. "That's the next big thing on my radar". I don't want to tell myself too much but since Augusta I have putted well.
He'll tee off in the final group at 1:45 p.m. "That is not where you want to be". "I've got this next week off to assess what I need to go forward". "I warm it up, it's OK", McIlroy said. "But take him out of the equation, I feel like I'm still right there in the tournament".
McIlroy said the pain wasn't as bad as it was early in the year at the Players, when he said it really flared up on him.
He birdied the fourth to turn in one under par, then converted from eight feet for birdie at the 10th, made a 20 footer at the 11th, saved par from 14 feet at the 12th, knocked in a seven footer for birdie at the driveable 14th and chipped dead to set up a tap in birdie at the par-five 15th to get two under. "18, I hit the best putt of the day and it didn't go in". Just sort of have a chat with him about it and see what we need to go going forward. If I was injury-free, that wouldn't happen.
"I can feel it".
The combined holes have played an average of 0.916 strokes above par, with the No. 2 finishing stretch being the Memorial Tournament's Muirfield Village, which has played 0.598 above par.
The leaders are not out for another couple of hours but in the meantime there are plenty of players further back hoping to slide up the leaderboard a few places and earn a few extra quid. "But the more I play, it's just not allowing that time to heal 100 percent". His concern is that he doesn't think he can contend on a regular basis unless he gets 100 percent healthy, and that's not where he is now.
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