Argentina's Emiliano Grillo and England's Matt Fitzpatrick are the joint-leaders after the first round of the Arnold Palmer Invitational in Florida.
Grillo, the 2016 PGA Tour Rookie of the Year, birdied five of the first 11 holes and two of the last three en route to his five-under 67.
Fitzpatrick went through bogey-free in the opening round of the event, the first since Palmer's death in September.
Paul Casey and American duo Lucas Glover and Charley Hoffman are tied for third, one shot back.
Florida-born Australian Ryan Ruffels, 18, and compatriot Greg Chalmers are at three under.
In unusually cool conditions at Bay Hill, defending champion Jason Day posted a two-under 70.
Making a much poorer start was Rory McIlroy, the four-time major champion producing a rollercoaster back nine to card a two-over 74.
The Northern Irishman was one under through nine, but he bogeyed the 10th before finding a bunker and water at the next for a double bogey.
He had birdies at 13 and 16, but bogeys at 15 and 18 saw him end up in a tie for 58th.Cody Gribble, who had a forgettable five-over 77, produced one of the moments of the day, nudging an alligator back into the water.
Don't try this at home.— PGA TOUR (@PGATOUR) March 16, 2017
Sharing 58th place with McIlroy is Palmer's grandson, Sam Saunders.
The "shot" of the day.— PGA TOUR (@PGATOUR) March 16, 2017
Saunders tees off with Arnie's cart in the foreground. pic.twitter.com/NBjdzzjcoA
Reigning Masters champion Danny Willett, meanwhile, was a late withdrawal due to illness.
|Messi at 30: Leo´s career in numbers|
|Orioles pitching gives up 15 more runs, Nationals and Royals claim walk-off wins|
|Fiji spoil Ford´s party in win over Scotland|
|MLS Review: Giovinco seals win as Toronto stay unbeaten at home, Texas derby ends 1-1|
|Cabrera wants Mets to trade him|
|Donnarumma? There´s nothing new – Fassone|
|Former West Ham and Madrid defender Arbeloa announces retirement|
|Big Papi´s big night: Red Sox retire David Ortiz´s number 34|
|Vegas Golden Knights add to developing roster with three first-rounders|
|Butler posts heartfelt thank-you letter to Chicago Bulls|
|Messi could´ve played for Real Madrid instead of Barcelona – Gaggioli|
|Fultz, Embiid fully believe Sixers are a playoff team|
|Oshie staying with Capitals on new eight-year deal|
|Devils take Hischier with number one pick in NHL draft|
|Spieth maintains Travelers lead as struggling Day misses cut|
|Porto´s Depoitre joins Huddersfield for club-record fee|
|Stokes slams Roy obstruction dismissal|
|Five championships sounds pretty good - Hamilton targeting more F1 titles|
|Portugal U21s won´t be matching the Euro success of Ronaldo and Co.|
|Morgan defends Livingstone after disastrous debut run-out in South Africa defeat|
|Hardaker shines as Tigers beat Rhinos, Salford suffer late heartache|
|Saints legend Le Tissier urges Pellegrino to entertain|
|De Villiers hails South African never-say-die attitude|
|Salah set to thrill Liverpool fans - Ince|
|Rain washes out opening ODI in Trinidad|
|Cilic and Dimitrov through to Aegon Championships semi-finals|
|Proteas pull off thrilling fightback to set up T20 decider|
|Ronaldo and Hernandez don´t compare for Vasin|
|Broad in contention for opening South Africa Test|
|Garcia beats Vinci to book Mallorca reunion with Sevastova|
|Schick lauded as Czech´s best ahead of Juventus move|
|Safarova survives gruelling Gavrilova bout, Kvitova awaits|
|Dortmund opportunity too good for Bosz to turn down, says De Boer|
|The party won´t end if we exit Confederations Cup, says Russia boss Cherchesov|
|Man Utd star Carrick: I might play for as long as Giggs!|
|Contador supported by Degenkolb in Trek-Segafredo Tour de France team|
|Confederations Cup Social Diary: Ronaldo not on winning side for once|
|Chile v Australia: Troisi looking at bigger picture for Socceroos|
|Garcia, Stenson hunting half-way leaders in Munich|
|Jagielka buoyed by Everton transfer business|
|Dan Evans confirms positive cocaine test|
|Germany v Cameroon: Logistical motivation for Low|
|Federer ends Mayer´s title defence in Halle|
|Alonso´s season continues to go downhill as car rolls away|
|Chelsea to Aston Villa will not feel like a step down for Terry, claims Sherwood|
|Pellegrino appointed Southampton manager|
|Paul George gave Pacers a ´gut punch´ with news of desired departure|
|Confederations Cup Diary: Mexico win MediaBowl, Beausejour is our new best friend and everyone loves|
|Salah signing takes Liverpool a step closer to title - Berger|
|Bumrah unimpressed as traffic police mock no-ball woe|
|F1 Raceweek: Mercedes and Ferrari vie for momentum in Baku|
|Frank de Boer will fulfil ´every manager´s dream´ at Crystal Palace, says brother Ronald|
|Kaymer likens Woods´ impact to Muhammad Ali|
|Bale determined to win ´more trophies´ as Madrid exit rumours persist|
|Osorio: Russia clash the perfect chance to answer critics|
|Josh Harrop explains Manchester United exit|
|Carrick not thinking about Cristiano Ronaldo reunion at Man Utd|
|Santos defends Portugal´s Euro 2016 win|
|Vinales puts down a marker as Lorenzo endures dismal day|
|Man Utd youngster Harrop signs for Preston|
|New Zealand v British and Irish Lions: Everything you need to know|
|Ferguson was stunned when Van Gaal sold Evans, claims O´Neill|
|Santos keeps New Zealand guessing on Ronaldo|
|PSG´s project is not lost – Marquinhos urges Verratti to stay amid Barcelona interest|
|Don´t compare me to Mourinho, says New Zealand boss Hudson|
|Leeds complete signing of Poland midfielder Mateusz Klich|
|Hudson claims New Zealand are not scared of Ronaldo and Portugal|
|Luke injured in Warriors win, Hayne torments Tigers|
|Sevilla refute Juve interest in N´Zonzi|
|River Plate duo Mayada and Martinez Quarta fail drugs test|