Adam Hadwin ended Thursday as the outright leader following the opening round of the U.S. Open in Brookline, Massachusetts.

The Canadian shot a four-under-par round of 66, one ahead of five players tied for second, including Rory McIlroy, who had been four under himself before bogeying his final hole on the ninth.

Callum Tarren, David Lingmerth, Joel Dahmen and M.J. Daffue sit alongside McIlroy, with seven more players on two under, including Justin Rose and Dustin Johnson.

It was otherwise not a great day for some of the LIV Golf International Series participants, with Phil Mickelson carding an opening round of 78 (seven over), while Louis Oosthuizen managed just one shot better and Sergio Garcia finished on four over.

LIV Golf's new additions Patrick Reed and Bryson DeChambeau ended even par and one over respectively. 

World number one Scottie Scheffler recovered from a wobbly start to finish on even par, while PGA Championship winner Justin Thomas ended the day one under, as did the man he beat in a playoff for that trophy, Will Zalatoris.

Defending champion Jon Rahm, Collin Morikawa and Adam Scott also shot one-under rounds of 69, while world number four Patrick Cantlay came away from Thursday two over.

Shot of the day

After ending up just off the green in the longer grass on the 12th, a precision chip from Matt Fitzpatrick still had a significant distance to travel, but slowly rolled its way straight into the hole to the delight of the Englishman and the Brookline crowd, sending him back to two-under straight after bogeying the 11th.

Player of the day - Adam Hadwin

Hadwin sat on one over after three holes, before birdieing five of the next six to catapult himself into the leading pack. The 34-year-old has never finished higher than T39th in this tournament, and also responded to a bogey at 12 with another immediate birdie at 13, and then ended with five tidy pars to head into day two as the outright leader.

Chipping in

Rory McIlroy: "I'm going into tomorrow with the mindset of 'let's keep it going', rather than 'where is the cut line' or whatever. If you don't get off to a great start, those thoughts start to creep in, 'okay, what do I need to just be here for the weekend?'"

Jon Rahm (asked about two children stealing his ball on the 18th hole): "Yes… I'm pretty sure I know who it was. I recognised the two kids that were running the opposite way with a smile on their face. (Laughing) I am 100 per cent sure I saw the two kids that stole it."

A little birdie told me...

- McIlroy's 67 was the 13th of his career at the U.S. Open, now level with Seve Ballesteros and Sergio Garcia for most by a European player at the tournament.

- Lingmerth, ranked 592nd in the world, has never finished worse than tied for 21st in three previous U.S. Open appearances, and the Swede started with a promising 67 here.

- The first round scoring average of the last 10 winners at the U.S. Open is 69.1, with 25 players hitting under that on Thursday.