Both men are firmly in the hunt for their first major championship win after American Zalatoris shot 67, the day's lowest score, and Fitzpatrick of England only needed a shot more.

2021 Masters runner-up Zalatoris came out fast with an impressive front nine, including birdies at holes two, four and nine, and just one bogey all round at hole seven propelled the 25-year-old to the joint lead.

In contrast, world number 18 Fitzpatrick's form was more erratic on the par 70 course, with a purple patch between holes 14 and 17 seeing him shoot three birdies out of four before finishing up with a bogey.

The total number of places under par at the tournament dropped from 23 to just nine, on a day of tricky conditions at The Country Club in Brookline.

"This place is a beast," Zalatoris said. 

"When I played during the [U.S.] Amateur in 2013, I said this was the hardest golf course that I had ever played. 

"It's just so easy to compound mistakes out here, which, of course, you can do that in major championships in general, but especially this one."

Spain's Jon Rahm, who won the US Open last year, lies one shot behind the pair on three-under-par after a double bogey on hole 18 halted his late momentum and left him on a score of 71.

Three players sit two under with one round to play, including world number one and new Masters champion Scottie Scheffler who got as low as six under before struggling between holes 11 and 14.

Despite shooting an underwhelming 73, Northern Ireland's Rory McIlroy, who won the Canadian Open last weekend, remains in the mix thanks to strong previous rounds on one under, alongside the United States' Sam Burns and Joel Dahmen.

The 122nd US Open Championship is expected to conclude today, with the final pairing of Zalatoris and Fitzpatrick due to tee off at 2.45pm ET.