The U.S. Open didn't seem to faze him, and he delivered a variety of big shots that led to his sensational finish.
Don't be alarmed to see a few names atop the leaderboard at the U.S. Open that are familiar only to hard-core golf fans.
This might have been more than anyone was expecting. He missed almost two months at the start of the year when he was diagnosed with a slight rib fracture, and then the latest break after The Players Championship.
Given the historic nature of this week's scoring - consider that from 1895 to 2016, just a single player finished at 11 under or better through 54 holes at the U.S. Open - maybe the more esoteric question is how many birdies are too many birdies?
But while Casey was relieved after his gutsy fightback, it was back at the drawing board for McIlroy and Day after their disappointing exits.
It was the first time since the world ranking began in 1986 that the top three players missed the cut in any major.
Joining them were British Open champion Henrik Stenson (No. 6), Alex Noren (No. 8), Jon Rahm (No. 10), Justin Rose (No. 11) and Adam Scott (No. 12). He didn't make a birdie the rest of the way and had to settle for a 70.
Fleetwood continued his consistent form in the third round at Erin Hills and shared the lead with playing partner Brian Harmon after the Englishman made three birdies in four holes from the 12th.
"It will be just fine", said Fowler, who plays in the third-to-last group on Saturday.
But after three birdies in six holes, he eagled the 667-yard par-five 18th, hitting his second shot to eight feet from 300 yards and rolling in the downhill putt.
After weeks of pouring his heart and soul into qualifying for the first U.S. Open played in Wisconsin, Madison's Steve Stricker was right where he wanted to be after the tournament's second round.
But let's use Reed as an example - with five PGA Tour victories, he's the most accomplished player atop the leaderboard next to Fowler, yet he's never finished in the top 10 in 13 previous starts in major championships. Only a half-dozen of them have even experienced Sunday contention in golf's biggest events.
Dress rehearsals are all very well but Rory McIlroy confessed that when the curtain went up he was unprepared and fluffed his US Open lines. Johnson birdied his opening two holes of the second round in his bid to first make sure he had a tee time today, and then try to make up ground on the lead.
Thomas knows about tournaments and leaderboards. More important, he was happy with how he played Friday and remained optimistic that he could carry that over to the weekend.
Harman, bidding to become the first left-hander to win a U.S. Open, is a victor on the PGA Tour this year while Thomas is the world number 13, a three-time victor this season and a member of the exclusive "59" club after breaking 60 at the Sony Open this year.
Four others are within three shots as the third round winds down. "It's a major, it means a lot to me", Johnson said. Harman struck me as the kid you knew in high school - short, not cool, trying to fit in. "The putter was hot on the front side to keep me in it, so it's all there, it's just a matter of putting it together and I'm not out of it". It's still a U.S. Open.
Despite a flurry of late birdies it was not enough to turnaround an erratic round littered with errors and missed opportunities.
But that's not all that makes the possibilities so limitless Saturday.
It also put him squarely in the mix to capture his first major. "We're definitely in good position for tomorrow". But I'm looking forward to getting the job done.
American Patrick Reed also flirted with the U.S. Open record, signing for a seven-under 65 to headline a pack of five at eight-under.
The bogey save on 18 certainly changed his day.