Rory McIlroy remarkably slumped to 10 over through 11 holes and playing partners Jordan Spieth and Phil Mickelson were also struggling badly as scoring proved fiendishly difficult on day one of the U.S. Open.

Together in a feature group at Shinnecock Hills, McIlroy, Spieth and Mickelson all endured significant misery on their first nine after starting at the 10th.

McIlroy was seven over at the turn, with Mickelson and Spieth both three better off, the latter having dropped four shots inside two holes.

On a typically penal U.S. Open layout, things soon got worse for the trio, 2011 champion McIlroy double-bogeying the first - his 10th - and dropping another shot at the next to be 10 over after 11.

Spieth and Mickelson were six and five over respectively, while the likes of Jason Day (+7), Charl Schwartzel (+6) and Bubba Watson (+5) were also faring poorly.

Yet as many big names struggled, Ian Poulter and Matt Kuchar were both two under, with Scott Piercy and Charley Hoffman the only other men in red figures.

The high-profile trio of Tiger Woods, Dustin Johnson and Justin Thomas were due out together at 1:47pm local time.