Thomas' nine-under got him into contention, but there's still work to be done.
Bolstered by a delicate, bogey-saving chip shot at the 18th green, Fleetwood posted a four-under-par 68 on Saturday to stand 11 under par, just one stroke back of American lefthander Brian Harman heading into Sunday's final round at Erin Hills.
The Englishman had a chance for a piece of the third-round lead at the U.S. Open as he lined up his third shot on the par-5 final hole: a 50-yard wedge from the bottom of the hill that led to the green.
There were 18 rounds in the 60s, starting with one for the record book.
And the prime beneficiary was Thomas, who catapulted up the leaderboard with his record-breaking nine-under-par 63. "I'd love to have another one of those", Thomas said.
"I've made plan and I've stuck to the plan", he said.
"That was the best shot I hit all day".
Thomas, however, is not alone in that pursuit. Casey shot a 3-over 75 on Saturday to drop to eight shots back behind Harman.
Not to be overlooked was Si Woo Kim, who captured The Players Championship last month.
That's something Day, the No. 3 player in the world, rarely says at a major.
The weather forecast will play a vital role...
"It feels good", said the 39-year-old, who has recorded three consecutive top-six finishes in the Masters but whose sole top-10 finish in 13 US Open appearances came at Oakmont a decade ago. But he missed a pair of 10-foot birdie chances in that stretch, and one of the birdies he made was a two-putt from 20 feet on the par-5 seventh. That trio is tied for second at 11-under par.
What would winning the U.S. Open do for him? "I'm not sure when it's going to sink in". "But you can't do anything about that stuff. If you hit it off the tee you can score".
Fowler, sharing a house with Thomas this week, was being left behind until he ran off three straight birdies and shot 68.
He then drilled a 300-yard second shot to around eight feet before rolling in his eagle putt to roars from the galleries.
Thomas, who shot a 59 and has three victories this season on the PGA Tour, also moved into the lead with his putt at 18. 14 and 15 to get to 12-under.
Harman, 30, doesn't seem to be feeling the pressure, though that could change Sunday.
"It just depends on the weather", Harman said. Even as the course slowly recovered from rain early in the week, reasonable scores were available.
"It's just golf. It's frustrating but I don't let it bother me".
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