The Englishman's Ryder Cup opponent Phil Mickelson crept up on the rails while Westwood, Adam Scott, world number one Tiger Woods and fellow American Hunter Mahan were slugging it out in the two final groups.
Mickelson finished with four birdies in his last six holes to post a three-under clubhouse total and his competition just melted away with Westwood, who began the day with a two-stroke lead over the field and a five-shot advantage over Mickelson, finishing joint third on one over.
"I didn't really play well enough today. I didn't play badly, but I didn't play great," said the 40-year-old, for whom eight of his 16 top-10 major finishes have been either been second or third.
"It's a tough golf course and you've got to have your 'A' game. I missed a few shots out there.
"Sometimes you play well and somebody plays a bit better and sometimes you play poorly.
"I didn't really do either and Phil obviously played well. He shot the round of the day, five-under par and birdied four out of (the last) six - that's a pretty special finish in a major championship.
"But you've got to play well to give yourself your own momentum, and I just couldn't get there today.
"I didn't do a lot wrong, I just didn't do enough right. I know what I've got to work on.
"I finished top three in a major championship. I would like to have won but you can't not take positives from top three in a major.
"I keep putting myself in contention."
When Westwood birdied the par-five fifth he led by three but bogeys at seven and eight - finding bunkers on both holes - cut his advantage to one and before long he was trailing.
"Going from three-under back to one-under just halted my momentum a bit," he added.
"I hit the wrong club at 7 I thought, in hindsight. I tried to hit a nine-iron and it was never going to get there.
"It plugged in the front trap (taking two attempts to get out) and plugged it in the trap at eight."
Westwood admitted he was surprised to be going into the final day as leader, never mind with a two-shot advantage.
But he stressed being in the last group, with the weight of expectation looking for the first English winner of the Open since Nick Faldo in 1992, did not over-burden him.
"For me to be last off in the Open Championship I think was a probably a new experience and I really enjoyed it," he said.
"It's where any professional golfer wants to be. It means a lot and you go out there and try your best but there was no pressure.
"I was amazed to be in the lead going into the fourth round, because every time I turned into the wind I was really struggling.
"I didn't feel like I was striking the ball well but I putted lovely this week and I made my fair share so there was a lot of positives to take out of the week.
"I'm not too disappointed. I don't really get disappointed with golf any more."