The Worksop man was well set, four under par going onto the 16th.
But bogeys at each of the last three holes left him one under par for the round.
He admitted a long delay, after a rules debate in a match up ahead, disrupted his rhythm, but insisted he’s still in contention for a first ever major title.
Speaking after his round of 71, the 42-year-old said: “(A) disappointing finish.
“Three bogeys to finish, but you know, 71 and by no means out of the championship.
Westwood said he stiffened up a little during the lengthy wait to resume play, when a child picked up JB Holmes’ ball, but admitted he should have coped better with the delay.
Fellow Notts golfer Greg Owen finished four under par, and said he was happy with a ‘solid’ round.
The Mansfield export said: “I got off to a great start, it’s been great conditions.
“A nice birdie on the first and then birdied the last and in between it was just really, really solid.
“Just one bunker, which you make bogey from, and you accept, that’s what’s going to happen.
“Other than that, just realy happy and just played nice.”
The weather is expected to deteriorate tomorrow, but Westwood said it’s part and parcel of The Open and he’s prepared for it.
“You just have to grin abd bear it and get on with it really,” he said.
“Mastering the conditions and also playing St Andrews experience is a key, as well.
“It helps to have gone out and played this golf course in tough conditions before.”
Westwood’s ISM stablemate Danny Willett is currently five under through 10 holes.
The Sheffield golfer birdied the first three holes and eagled the par-four 10th to sit in tied seventh.
American Dustin Johnson is the clubhouse leader on seven under.
Westwood tees off at 2.12pm tomorrow, with Owen starting his second round at 11.33am and Willett off at 8.11am.