Casey collected three birdies on each nine at Spyglass www.pebblebeach.com/golf/spyglass-hill-golf-course and made his only bogey in the third round on No. 6 to record a 54-hole total of 15-under 200.
Mickelson, who hit every fairway in his first round for the first time in nearly 21 years, made a strong start to day two in the cool and damp conditions as he birdied the long 11th - his second - and then hit a remarkable three-wood to eight feet from the trees at the par-five 14th, although he mis-read the eagle putt and tapped in for birdie.
California native Phil Mickelson feels his intimate knowledge of poa annua greens will give him a distinct advantage as he chases a 50th career victory at the Pebble Beach Pro-Am.
Mickelson, among the leaders starting the round, was even for the day through nine after three bogeys and three birdies.
"It was nice to birdie the last, but I hate finishing the round off like that - three bogeys after having a pretty good round going in".
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"I thought anything in the 60s was going to be a heck of a round, and I was one shy", Mickelson said. "So I've been able to make a few more birdies on some of these holes". "Obviously Paul is playing some great golf".
Jason Day leads the Australians after his round of 72 today at Pebble Beach saw him lose ground but he is six shots behind the leader and just three shots out of second place so something very special from him tomorrow could yet see him contending for the title.
Former US Open victor Lucas Glover (70) and fellow American Scott Piercy (69) share third at 11 under. "He's going to be tough to catch".
Strong winds and an occasional rain squall made for testing conditions on Saturday, and Casey acknowledged that the weather created an element of luck.
Jordan Spieth, who was tied for the lead through 16 holes when rain halted the second round on Friday, returned with a late bogey to start the third round one off the pace.