Wyndham Clark’s lead shrank, then grew, then all but swallowed the tournament whole. The 2023 US Open champion watched a four-shot advantage get cut in half on Saturday while still on the first hole, only to respond with a masterclass in survival golf as Shinnecock Hills finally delivered the bruising examination players had anticipated all week.

By day’s end, Clark had stretched his lead to a yawning six shots despite shooting an even-par 70. Scottie Scheffler’s one-under 69 was enough to emerge as the closest pursuer, but the world No 1 will begin Sunday’s final round needing something extraordinary to prevent Clark from capturing America’s national championship for a second time in four years.

Clark arrived at the weekend at seven under par after setting the 36-hole scoring record for a US Open at Shinnecock. For a brief moment on Saturday afternoon, that margin looked vulnerable. Sam Stevens birdied the opener while Clark’s approach spun back down the false front of the first green. A tentative first putt left him six feet for par and the return effort slid past the edge.

A four-shot 36-hole lead had become a two-shot 37-hole lead. With winds approaching 40mph sweeping across the exposed property and the Poa annua greens growing firmer and increasingly unpredictable, the red scores began dropping off the leaderboard one after another.

But Clark never came close to joining them. The 32-year-old American birdied the par-five fifth and spent much of the afternoon producing the sort of gritty, unglamorous golf that wins the major championship billing itself as golf’s toughest test. He repeatedly escaped trouble with timely par saves, converting putts from 5ft on three occasions while also rescuing pars from six, seven and 14ft.

“That’s what you have to make to win US Opens,” Clark said. “You’re not going to have too many birdie putts … you’ve got to make those kind of five- to 12-footers.”

Clark had spent the afternoon slipping Shinnecock’s punches, but the 16th was where he landed what may have been the knockout blow. A towering 275-yard approach at the par-five settled inside 5ft of the flag, setting up the first eagle of the week at the hole and effectively slammed the door on the field.

The exposed layout, roughly 80 miles east of Manhattan, played firmer and faster than during the opening two rounds. Of the 10 players who began the day under par, only five finished there.

Rory McIlroy was not among them. The Masters champion appeared ready to mount a serious challenge after producing three consecutive birdies from the fifth hole, including a remarkable 66ft putt from off the sixth green. The surge moved him to two under par and within striking distance of Clark.

Then everything came undone. McIlroy’s approach from just 49 yards at the 10th bounded through the green and led to bogey. A three-putt followed at the 12th. Further mistakes arrived at the 14th, 15th and 18th as a promising round dissolved into a three-over 73. He left the course without speaking to reporters.

While McIlroy and others drifted out of contention, Scheffler marched steadily in the opposite direction. The four-time major and Olympic champion looked to be fading from the mix after opening with back-to-back bogeys, but a birdie at the 10th sparked the turnaround. Scheffler then birdied three consecutive holes from the 14th, holing a 65ft chip from off the green before adding a 12ft birdie putt at the next and narrowly missing eagle at the par-five 16th.

A bogey at the short 17th and a missed birdie chance from 4ft at the last prevented an even lower score, but his 69 was still the best round among the leading contenders and left him alone in second place at one under par.

Sunday’s final round falls on Scheffler’s 30th birthday and Father’s Day and victory would complete the career grand slam. Having already captured the Masters, PGA Championship and Open Championship, he would join Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus, Tiger Woods and McIlroy as the only men to win the sport’s four bedrock tournaments.

“I’d rather be leading,” Scheffler said. “But I have an opportunity to go out there and have a great round and give myself a chance to win the tournament.”

Stevens, in a four-way share of second at one-under with Scheffler, Tom Kim and Sahith Theegala, continued one of the week’s most surprising performances. The 29-year-old Texan, playing only his eighth major championship and still in search of his first PGA Tour victory, remained firmly in the mix after another composed display. The former US Open champion Matt Fitzpatrick, who began the day tied for second, slipped backwards with a 74.

Earlier, Emiliano Grillo posted the day’s low score with a three-under 67. The Argentinian became only the second player this week to make four consecutive birdies, matching Dustin Johnson’s feat on Friday, and joined a group of three players at even-par for the championship that included Xander Schauffele and Sam Burns. But all of them are chasing Clark, who first moved into the lead at 7.09pm on Thursday evening and has not relinquished it since.

His six-shot advantage is the third-largest 54-hole lead held by a US Open leader since the second world war. History suggests it will be enough: 21 players have carried a lead of six shots or more into the final round of a major championship with 20 of them going on to win. The lone exception remains Greg Norman’s collapse at the 1996 Masters, where a closing 78 transformed a six-shot advantage into a five-shot defeat against Nick Faldo.

“Scottie is the best player in the world, and he’s going to play probably really good. He always does,” Clark said. “But it’s nice to have a six-shot lead on him.” He added: “I’m not necessarily thinking about my lead or anything. If I go out and execute and go through my process and hit the shots I know I can hit, I like my chances.”

Tournament officials announced a record-equalling $22.5m purse on Saturday, with the winner set to receive $4.5m.