McIlroy carded a three-under-par 67 at Brookline to share second place with England's Callum Tarren, Swede David Lingmerth, American Joel Dahmen and South Africa's M.J. Daffue, a shot behind Canada's Adam Hadwin.

A month after an opening 65 in the PGA, it was another welcome good start to a major for McIlroy after previous struggles, but it was not without incident as he slammed one club into a bunker and threw another down the fairway on his final hole.

After starting from the 10th, McIlroy had covered his first 13 holes in two-under-par before a pushed tee shot on the 5th finished in thick rough on the edge of a bunker and left him struggling to take a stance in the sand, with the ball well above his feet.

 "The margins are just so fine in this tournament, and I think you can sort of see that out there with some of the reactions". - Rory McIlroy.

From there McIlroy could only hack his second shot into another bunker a few yards in front of him, prompting a furious reaction which saw him twice slam his club into the sand.

"You're going to encounter things at a U.S. Open that you just don't really encounter any other week," said McIlroy, whose mood was not helped by the slow pace of the group in front featuring Hideki Matsuyama and Xander Schauffele.

"It's hard not to get frustrated."

Having saved par at the 5th, McIlroy birdied the 7th and 8th to take the outright lead, only to drop his only shot of the day on the ninth after missing the green with his approach.

That prompted the club throw and McIlroy added: "The margins are just so fine in this tournament, and I think you can sort of see that out there with some of the reactions".