The former European number one fired his second two-under-par 70 of the tournament but a three-over 75 sandwiched in between has stymied his bid to make a move on the players at the top of the leaderboard.
Westwood said he had been fighting a hook throughout the event and had missed too many short putts.
"I'm just hitting a bit of a pull now and again which is destroying me," the 39-year-old Englishman told reporters after finishing on one-under-par 215 following a day of swirling 40-kph winds at a sun-baked Wentworth.
"It's my bad shot when I'm not swinging very well and I'm not swinging very well at the moment. It's a case of doing a bit more practice.
"I'm a bit tired. I've played a lot this year already and there's been a lot of going backwards and forwards (across the Atlantic)."
Several times one of the bridesmaids and never yet the bride in major championships, Westwood has once again been haunted by a cold putter at the European Tour's flagship event.
"Over the week I've just missed a few too many short putts," he explained. "I missed three times inside five feet today at the eighth, 10th and 12th."
Westwood amassed six birdies on Saturday but a triple-bogey seven at the 13th proved extremely costly.
"I hit it in the fairway trap on 13 and my escape plugged in the face," he said. "I then dropped the ball in the bunker, it plugged again, hit a seven-iron to 30 feet and holed it.
"Then I hit a five-iron at the next and lipped out for a hole-in-one."
A BMW is on offer for an ace at the 179-yard 14th and Westwood said it was a prize he would have cherished.
"I looked at it yesterday, I had a feeling good things might happen to me at that hole," said the Ryder Cup stalwart. "I like that car, the steering wheel is on the right side and it's got nice white leather."