Westwood had seemingly done the hard work by getting through 16 holes in level par for the day, despite playing the vast majority of those in heavy rain.
But the 40-year-old Englishman then ran up a double-bogey on the eighth and dropped another shot on the ninth to card a 73 that left him one-under- par at the halfway stage.
That was three off the clubhouse target set by former US Open champion Webb Simpson and matched minutes later by Ryder Cup hero Martin Kaymer, Simpson equalling the course record with a 64 and Kaymer carding a second consecutive 68.
Masters champion Adam Scott was seven under with one hole to play with US Open champion and playing partner Justin Rose five under after five birdies in eight holes.
“I played pretty well,” said Westwood, who is still seeking his first major title at the 63rd attempt after letting a two-shot lead slip going into the final round of the Open at Muirfield last month.
“The course was playing really long, the par-four 17th was unreachable, and I was just ticking along before getting kicked where it hurts on the last two holes. I would like to have finished par, par but I am in red figures going into the weekend and still have a chance.”
Westwood teed off in light drizzle and made regulation pars at the 10th and 11th before the rain became progressively heavier to threaten another stoppage in play after yesterday’s 70-minute delay.
That did not stop the former world No 1 from rolling in a 25ft birdie putt on the 12th however, and although he was unable to birdie the par-five 13th for the second day running – he missed from four feet immediately after the stoppage on Thursday – he did pick up a shot on the next to join Scott in the lead.
Scott regained top spot when his birdie putt from 15ft on the 13th just had enough pace to trickle into the hole, and he was soon two clear as Westwood dropped his first shot of the week on the 17th. Seeking to become the first man since Jack Nicklaus in 1975 to win the Masters and US PGA in the same year, Scott then moved three ahead with a birdie on the 16th, only to follow Westwood in dropping a shot on the 17th.
Defending champion Rory McIlroy had safely made the halfway cut thanks to a brilliant recovery, the world No 3 adding a 71 to his opening 69 to finish level par.
McIlroy has been watching videos of his eight-shot triumph at Kiawah Island last year for inspiration but it did not look set to pay off when he covered the back nine in 39 after three bogeys and a double-bogey on the par-3 15th. Another bogey on the first had the 24-year-old in real trouble but he birdied the third, fourth, seventh and eighth to avoid a second straight missed cut in the majors.