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.
|Kerr: Ginobili should keep playing another two years|
|He keeps getting better and better - Henderson full of praise for PFA winner Salah|
|Roma v Liverpool: Giallorossi have chance to write positive headlines|
|Prove you are the best - Povetkin promoter lays down Joshua gauntlet|
|Wenger will not be short of offers from clubs - Dein|
|LeBron hails Korver & Clarkson as Cavs square series|
|I may play the kids in the FA Cup next season, reveals bemused Pochettino|
|Simeone weighing up three-man defence for Arsenal tie|
|Kings wait for Bogut signing approval|
|Vieira refuses to address Arsenal rumours and Wenger departure|
|Cueto lowers historic ERA in Giants´ win|
|Spurs avoid sweep, Cavs edge Pacers|
|WATCH: Smith beats first-quarter buzzer with near full-court shot|
|Harden, D´Antoni hope Rockets regain top form|
|Man United star Pogba can win Ballon d´Or, says Ronaldinho|
|Silva loves me more than his girlfriend – Mendy|
|MLS Review: Timbers end NYC´s unbeaten start, Sounders claim first win|
|Flyers´ Couturier played with torn MCL|
|Emery praises PSG for maintaining motivation|
|Cavs´ Hill out for game four with back spasms|
|Oblak unsure over Atletico Madrid future|
|Klopp ready for ´different´ Roma test|
|Salah always determined to prove his Premier League worth|
|Landry earns first PGA Tour victory at Texas Open|
|City´s Sane named PFA Young Player of the Year|
|Salah pips De Bruyne to PFA Players´ Player of the Year award|
|Napoli bus spat at and insulted, Sarri claims|
|Navas will be at Real Madrid for many years – Perez|
|Juve still have the advantage, Allegri claims|
|Racing coach Labit ready for ´formidable´ Leinster challenge|
|Smalling not giving up on World Cup|
|Hard to catch Man City if they invest, Mourinho warns|
|Pogba: I have no problem with Mourinho|
|Koulibaly: Napoli have always believed in Scudetto|
|Bordeaux 0 Paris Saint-Germain 1: Lo Celso keeps champions on track for records|
|Atletico Madrid 0 Real Betis 0: More concerns for Simeone ahead of Europa League|
|Juventus 0 Napoli 1: Partenopei close the gap thanks to Koulibaly|
|Dominant Marquez maintains perfect record in Austin|
|Guardiola has no problem with Manchester City pitch invasion|
|Neymar must join Real Madrid to become the best - Rivaldo|
|There was no player better than De Bruyne - Guardiola backs City man for PFA award|
|Top managers should want Arsenal job, says Moyes|
|Manchester City boss Guardiola aiming to smash records following title triumph|
|Las Palmas relegated from LaLiga after 4-0 home defeat to Alaves|
|Archer and Gowtham combine for dramatic Royals victory|
|This won´t happen forever - dominant Nadal reflects on latest success|
|Andrew Bogut returns to Australia, signs with Sydney Kings|
|Manchester City break Premier League passing and possession records|
|Conte: Morata still important despite Giroud impact|
|Inter gave up against Chievo, admits Spalletti|
|Conte denies Mourinho rift ahead of FA Cup final|
|Levy claims fifth European Tour crown in Rabat|
|Diouf miss ´a turning point´ for both Stoke and Burnley|
|Hughes upset with lack of VAR after Southampton´s FA Cup loss|
|Manchester City 5 Swansea City 0: Guardiola´s champions turn on the style|
|Manchester United are FA Cup favourites, claims Chelsea boss Conte|
|United States and Czech Republic reach Fed Cup final|
|Rapid start sees Racing through to Champions Cup final|
|Benevento relegated after late Crotone winner|
|Jungels roars to Liege-Bastogne-Liege victory|
|I don´t need to die anymore - Wenger reflects on Arsenal ´funeral´|
|Hazard gives Conte a dazzling reminder of better days|
|Playing at Wembley is like playing in my garden - Giroud|
|Sevilla president casts doubt on future of Montella|
|´Hurtful´ fans damaged Arsenal´s image, Wenger claims|
|Chelsea 2 Southampton 0: Giroud and Morata book FA Cup final spot|
|Super Kings survive late onslaught to overshadow Sunrisers|
|Struggling Stoke have to win at Liverpool, concedes Butland|
|Did Wenger jump or was he pushed? – Arsenal boss proves evasive|
|Ramsey: Arsenal owe Wenger the perfect finale|