Defending champion Paul Casey fired a five-under par 66 to grab a share of the 36-hole lead alongside American Austin Cook at the US PGA Tour Valspar Championship on Friday.
Knox hit a 3-wood 275 yards for his second shot on the par-5 11th hole that ran on and into the cup. Luke Donald, who briefly held the lead, shot a 70 and was one behind, along with Scott Stallings (68) and Sungjae Im (67).
Casey, who edged Tiger Woods and Patrick Reed by a stroke a year ago for the crown, began Friday on the back nine and promptly birdied three of his first four holes, sinking a 10-foot putt at 10 and landing his third shot inches from the cup before a tap-in at the par-5 11th.
Donald, playing in only his second tournament of the season after back trouble in 2018, overtook Casey and Cook with a clutch 33-foot birdie from a greenside bunker at the fifth, his 14th hole.
Casey and Cook, who shot a 67, were at 6-under 136.
"I've never (successfully) defended a professional event", he told Golf Channel. "Quieted the transition at the top of the swing", Casey said.
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"Maybe lost a little bit of focus down the stretch there the last few holes, but overall I'm in a great position and thankful that I feel good and healthy and ready for a weekend".
But Casey's lone bogey came on his closing ninth hole, finding bunkers with his first two shots and missing a six-footer for par.
"Disappointing on nine", Casey said.
Cook turned to a new putter and liked the results as he recorded five birdies and a bogey.
Australian Jason Day did fall over the cut line as the world number 12's 71 was not enough to overcome an opening 74. Along with the two bogeys he carded, Johnson had three birdies and made some impressive shots.
Coming off a victory three weeks ago in Singapore, the top-ranked Park birdied five of the first six holes on her final nine.