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.
|Chase hit with two-year ban for cocaine use|
|Jones returns to lead much-changed Wales against All Blacks|
|Chelsea´s best chance of success is Champions League, claims Lampard|
|F1 Raceweek: Hamilton not relishing imminent Halo, Vettel wants Baku awards|
|Cummins savours taking ´dream´ Root wicket|
|Jones makes nine England changes as Simmonds starts at number eight|
|Robben: It could be over for me after this season|
|Newcastle Jets 4 Melbourne Victory 1: Nabbout at the double|
|Chawrasia leads from Fitzpatrick in Hong Kong|
|Vince revels in silencing the doubters|
|Happy Sanchez could win the Premier League for Arsenal, Lauren claims|
|Davis leads Australian Open as Spieth struggles in the wind|
|Halep tips Dimitrov for grand slam glory|
|The Ashes round-up: England falter after Vince and Stoneman half-centuries|
|Angered Neymar storms away from Real Madrid questions|
|Bayern played their worst ´for a long time´ - Boateng|
|Australia fight back on tight first day in Brisbane|
|Neymar form no surprise for Alves|
|Fiji ready for another mission impossible - Naiqama|
|Vince helps England continue fine start|
|Thunder roll as Westbrook goes off against Warriors, Cavs win|
|Kiwis would consider Bennett as coach|
|Meninga unhappy with Aussie preparations ahead of World Cup semi|
|Federer congratulates Basel after Man Utd scalp|
|Heat snap Celtics´ 16-game winning streak|
|Spurs star Leonard has unique issue, Popovich says|
|England steady after early Cook loss in Brisbane|
|Gremio 1 Lanus 0: Cicero gives Brazilians advantage in Libertadores final|
|Hazard: I admire Madrid, but I´m happy at Chelsea|
|Neuer or De Gea? Digne says Ter Stegen is world´s best keeper|
|Allegri satisfied with draw against ´extraordinary´ Barcelona|
|Nainggolan reaffirms Roma commitment after Simeone praise|
|Chiefs sign veteran CB Revis|
|England choose to bat as Warner, Marsh cleared to play for Australia|
|Beathard, not Garoppolo, named 49ers starter v Seahawks|
|Simeone hails Griezmann´s ´extraordinary´ volley|
|Emery hails Neymar after stunning Celtic win|
|Freeney joins Lions day after being cut by Seahawks|
|Championship Review: Wolves thrash 10-man Leeds, Middlesbrough cruise|
|Goncharenko delighted for clean sheet star Akinfeev|
|I hope Barcelona rest Messi – Jesus|
|Broncos name Lynch starting QB for Raiders game|
|Thiago suffered a serious injury, says Heynckes|
|Valverde explains decision to bench Messi|
|Coleman the latest to join Rebels revolution|
|LeBron: Defensive consistency key to Cavs winning streak|
|Bills revert to Tyrod Taylor as starting QB after Nathan Peterman disaster|
|Absent Stokes wishes England well|
|Iniesta hails ´splendid´ Ter Stegen as Barcelona seal last-16 spot|
|Griezmann shrugs off goalless streak after wonderstrike keeps Atletico´s hopes alive|
|We should have been 5-0 up – Mourinho unimpressed with wasteful United|
|Toure reveals Guardiola anger with City´s Feyenoord display|
|Warriors star Durant says he will play against Thunder|
|Paris Saint-Germain set Champions League scoring record in Celtic rout|
|Sporting CP 3 Olympiacos 1: Dost on target as hosts stay in contention|
|Bielsa suspended by struggling Lille|
|Anderlecht 1 Bayern Munich 2: Lewandowski and Tolisso give visitors win|
|Atletico Madrid 2 Roma 0: Griezmann returns to form with wondergoal|
|Basel 1 Manchester United 0: Late Lang goal leaves Mourinho´s men with work to do|
|Paris Saint-Germain 7 Celtic 1: Cavani and Neymar score braces in sparkling win|
|Juventus 0 Barcelona 0: Messi benched but Valverde´s men in last 16|
|Bennett backs New Zealand to recover from World Cup exit|
|No Samoa donation from England squad - Borthwick|
|Conte pleased to take pressure off Atletico clash|
|Success in Europe will help Milan´s league form, Montella claims|
|Bauza sacked by Saudi Arabia|
|Browns´ Josh Gordon returns to practice|
|Azarenka confirms place at ASB Classic|
|Willian: For me, it was a penalty|
|Akinfeev keeps first Champions League clean sheet since November 2006|