By Larry Fine
After an eagle-eagle-birdie barrage by Mickelson starting at the par-five 13th, Westwood's four-shot lead turned into a one-stroke deficit in a span of just over 30 minutes.
"(It) seemed quicker," Westwood told reporters.
But the Briton stayed even-tempered as Mickelson made a bogey on 17 and he emerged as the leader with a one-shot lead over the American going into Sunday's final round.
"I've been proud of myself all week," said world number four Westwood, who shot four-under-par 68 for a 12-under 204 total.
"I've hit hardly any poor shots. I've hit a lot of good shots and controlled my emotions well.
"It was amazing how calm I felt out there today. I've got my own little bubble in my own little world that I wander around in now."
The Englishman, who tied for third in the last two major championships, knew he was part of something special on Saturday.
"It was probably one of those great days in golf," Westwood told reporters about Mickelson's scoring explosion that set off roars that served as a second-hand, play-by-play for him.
"There was no need to look at the scoreboard," he said.
"You have a fair idea where people are on the golf course. I heard a cheer when I was coming down 11, I figured he made the crowd behind the 12th there cheer when he made eagle.
"That's what major championships are about. They are tough ones to win, because great players do great things at major championships."
Westwood said he would maintain his same approach on Sunday when he tees off with Mickelson, with world number one Tiger Woods and South Korean K.J. Choi four shots back.
"Same as it was today," said Westwood. "Ignore them and just play my own game."
(Editing by Greg Stutchbury)