Howell - injured while warming up (Getty Images)

Ryder Cup partners David Howell and Paul Casey both withdrew from the US Open before their second rounds at Pinehurst.

It was no surprise to see Casey not returning after a horror 15 over par 85 had left him last of the 156-strong field overnight.

He is in the worst slump of his career at the moment, 46 over par for his last seven rounds, and did not give a reason for pulling out.

Howell, meanwhile, was getting ready to resume on four over when he suffered an abdominal muscle injury.

"Everything was fine and dandy, as they say, and then I felt a twinge," he said.

"Nothing untowards really and I thought it would wear off.

"Then I hit a three-iron and I just got what felt like a rip.

"I've never had this happen in 20 years of golf and there were no signs of it physically this week at all.

"But I hit it in the rough nine times yesterday and hacking out tightened things up a little bit and this morning just put it over the edge.

"It's very annoying. I've had a few injuries over the years, but nothing ever like this.

"It's unbelievably frustrating considering that I might be out for a few weeks right in the middle of the season.

"Hopefully, fingers crossed, it won't be as bad as it feels it might be."

Casey also opened with 85 in the 2002 US PGA at Hazeltine and the 2003 Open at Sandwich, but played his second round on each occasion.

He had spoken on Tuesday of feeling more upbeat about his game than of late, but come the tournament proper he had one birdie, five pars, nine bogeys and two double bogeys before finishing on the 175-yard ninth with a triple bogey six.

Casey hit only five fairways and was on just six of the 18 greens in regulation figures.

Howell, 11th on his debut in the Masters in April, lost recent play-offs in the British Masters and Irish Open, but has only bad memories of the US Open so far.

The only other time he qualified was in 2002 and he shot 78 and 81 not long after returning from a broken arm suffered when he tripped while jogging.

Casey has now failed to make the last two rounds three times out of three at the event and has made only one cut in six starts on the US Tour this season.

Casey's failure to cite an injury or illness for his scratching had the United States Golf Association's director of communications Marty Parkes saying: "To get into the championship is a highly-valued award and we aren't happy when someone pulls out without having a genuine medical reason.

"We don't tend to fine players, but we will take the opportunity to review what has happened and act accordingly."