Jason Day produced a superb back-nine to snatch the outright lead as Rory McIlroy endured an error-strewn opening day of the Arnold Palmer Invitational.
Day carded three birdies and an eagle in a sparkling five-hole stretch on the inward half, coming home in 31 to return a six-under 66 and upstage his inform compatriot Adam Scott at Bay Hill in Orlando.
Scott had earlier earned a share of the early clubhouse lead after firing a flawless 67, although he insisted he was not at his best from tee to green and had to rely on his short-game to scramble a number of pars.
But McIlroy faces a battle to avoid a weekend off after he opened with a wretched 75, pulling his first tee shot out of bounds while also putting three balls into the water.
Day's round was not without mishap as he hooked his drive at the ninth beyond the out-of-bounds markers and ran up a double-bogey six which brought an abrupt halt to a run of four birdies in six holes.
The world No 2 steadied the ship with a pair of pars before getting back on track with a delightful up-and-down from sand for birdie at the 12th, and he added another at the next following a pure approach to three feet.
Day rolled in an 18-foot putt for a further gain at 15, and he smashed a superb long-iron to 10 feet at the long 16th and converted the eagle chance to move to the top of the leaderboard.
Scott started at the 10th and opened with six pars before bringing his round to life with an even better approach to 16, holing from eight feet for eagle, and he then made a rare birdie at the 18th to turn in 33.
The in-form Scott, bidding to emulate the likes of Tiger Woods and Rory McIlroy by winning a third consecutive PGA Tour event, picked up further shots at the fourth and ninth to join fellow Aussie Marc Leishman and Brendan Steele in the clubhouse on five under.
Henrik Stenson soon made it a four-way tie at the top when he produced a fantastic finish, reeling off three consecutive birdies from the sixth before a cast-iron par at the ninth - his last - capped a blemish-free day.
Troy Merritt made three birdies in four holes around the turn as he also returned a 67, while English pair Justin Rose and Paul Casey were among 10 players to open with a 66.
Rose had two eagles on his card, including a thrilling hole-out from 112 yards for a two at the par-four third, while Casey birdied two of his first three holes and picked up two further shots as he went bogey-free.
Their fellow Englishman Chris Woods posted a solid 69 along with big-hitting Scot Martin Laird, but McIlroy slipped nine shots off the pace after his long-game deserted him.
McIlroy's mistake off the first tee cost him a double-bogey six, but he did well to save par after yanking his drive at the third into water as he holed a confident putt from just inside 20 feet.
He clawed a shot back at the sixth, although he missed a great chance for eagle after knocking a remarkable 248-yard three-iron to 10 feet, but he stumbled to another double-bogey at eight after pulling his second from a fairway bunker into the drink.
A neat up-and-down for birdie at 12 was offset by a scrappy bogey at the next, and his ambitious second to the long 16th did not come close to carrying the water, but he avoided further damage when he pitched to six feet and nailed the putt.