The round left him joint 43rd of the 54-strong field and 10 adrift of leader Jose Manuel Lara, but still in pole position.

Padraig Harrington, who needs a top-three finish, is only 30th and David Howell and Robert Karlsson, who have to be first or second on Sunday just to have a chance, are 14th and seventh respectively.

All the attention on the eve of the event had been on third-placed Howell, who was “50-50 at best” just to tee off because of his shoulder problem.

While the Swindon golfer was delighted with a one-under 70 in the circumstances, Casey was more concerned about his health than his score.

“I just want to get inside,” was the first thing he said after signing his card.

After a visit to a toilet, Casey added: “It was the worst I’ve ever felt on a golf course. I was up a few times last night and although it was not too bad starting out. I was not feeling good.

“Then it just got worse and worse. My stomach really cramped up on the 10th tee and on the 11th I got an injection to stop the nausea and a pill to solidify the rest.

“It feels like ’flu because my whole body is aching. In any other circumstances I don’t think I would have kept playing, but I’m not a fan of pulling out and I think I should keep going as long as I can or a doctor says otherwise.

“I had parma ham and melon and some pasta last night, but I don’t know whether it was that. Two caddies were feeling bad this morning as well and one of them was on a drip.

“A bread roll is all I’ve had today. I felt close to being sick a couple of times and I was just trying to get in without making a mess.

“I wasn’t even thinking about the score. I’m a long way back, but I’ll just try to get under par by the end of the week.”

On several occasions he crouched down and leant on his golf bag for support, but he soldiered on and still managed to beat nine others, Ryder Cup team-mate and eight-time European number one Colin Montgomerie among them.