He sits six shots behind clubhouse leader Paul Casey, with the England veteran shooting an impressive 67 at the hard Spyglass Hill course to climb to 15-under the card.
And PGA Tour debutant Ho Sung Choi is almost-certain to miss out on playing the final 18 holes at Pebble Beach after the unorthodox South Korean littered his card with five bogeys against only two birdies in a frustrating 75 which saw him slip to four over par for the tournament.
Mickelson was part of the celebrity rotation at Pebble and put on a show on the back nine, stuffing a wedge to 3 feet on No. 13, and following with a 4-iron to 4 feet on the par-5 14th hole for eagle.
"I don't have to do anything insane tomorrow to win the tournament, I would say", Day said.More news: How to watch Monday coverage of AT&T Pebble Beach Pro
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Three later became four when Casey completed a bogey-free 64 at Monterey Peninsula, the Englishman flying up the leaderboard when he made four birdies and one eagle in a sparkling six-hole run on the back nine, although he managed only one more gain at the sixth as conditions deteriorated.
Paul Casey, of England, follows his drive from the 18th tee of the Spyglass Hill Golf Course during the third round of the Pebble Beach Pro-Am.
Like a scramble out of the pocket to turn a broken play into a touchdown, Romo lined up his next shot and pulled off what one golf media outlet has already dubbed the shot of the year.
Given they finished between 60th and 71st, Scott and Senden made the event's 54-hole cut but not the final round. He also lost a two-shot lead in the 2017 Tour Championship to Xander Schauffele. Mickelson had consecutive bogeys as the wind and rain were at its worst along the coastal holes of Nos. "But I've got a good chance going into tomorrow".