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.
29 de mayo 2017
|I´m crazy about returning to Spain – Nolito eyes City exit|
|Roma defender Emerson suffers anterior cruciate ligament injury|
|Zenit sack Lucescu after missing out on Champions League|
|Kisner holds off Spieth, Rahm to conquer Colonial|
|This was probably my last game for Benfica - Ederson hints at Man City move|
|Empoli relegated as Crotone beat Lazio to secure Serie A survival|
28 de mayo 2017
|Lorient 0 Troyes 0 (1-2 agg): Goalless draw sees visitors promoted to Ligue 1|
|Dzeko´s 29 goals claim Serie A Capocannoniere|
|Sato wins crash-filled Indy 500, Alonso retires|
|Contrasting emotions for Kvitova and Kerber on day one in Paris|
|Inter 5 Udinese 2: Perisic stars in crushing victory|
|Ausilio confirms Manchester United interest in Perisic|
|Totti: I´m not ready to say it´s over|
|Alonso suffers cruel retirement in Indy 500|
|Thiem off to a flier at Roland Garros, Haase lies in wait for Nadal|
|Totti is not normal - De Rossi emotional after Roma icon´s farewell|
|Twenty-five seasons, 307 goals, one incredible legacy – the numbers behind Totti´s remarkable career|
|Quintana satisfied with Giro podium, hails deserved winner Dumoulin|
|Sampdoria 2 Napoli 4: Comprehensive win not enough to snatch second place|
|Roma 3 Genoa 2: Perotti seals Champions League berth in dream farewell for Totti|
|Leave him alone! - Montella shrugs off Donnarumma error|
|Scott Dixon escapes injury in spectacular Indy 500 crash|
|England rest Stokes, Woakes and Moeen amid injury worries|
|I didn´t want to leave Real Madrid - Ronaldo|
|Lions 2017: Why Warren Gatland´s men will beat the All Blacks|
|Lions 2017: Why the All Blacks will tame the Lions|
|Record-equalling Noren claims stunning Wentworth win|
|Button given farcical grid penalty that he will never serve|
|England youngsters impress against lacklustre Barbarians|
|Lions rampant despite Coetzee red card|
|Raikkonen struggling to understand Ferrari´s Monaco strategy|
|Incredible Dumoulin usurps Quintana to snatch Giro victory|
|Wehrlein: Button made ´a silly move´ in Monaco crash|
|Hamilton happy to take points and move on|
|The offside rule is crazy - Cahill hits out at Sanchez FA Cup goal|
|I think Rooney wants to leave Manchester United, says Robson|
|Cagliari 2 AC Milan 1: Pisacane late show seals fitting Stadio Sant´Elia farewell|
|I surprised myself! - Ferrari´s Vettel revels in Monaco Grand Prix glory|
|Inter announce Palacio departure|
|Kerber relieved to be leaving clay behind|
|Courtois ready to sign new Chelsea contract|
|Everyone wrote me off - Critics inspired Mertesacker before Arsenal triumph|
|Elgar left red-faced as Somerset collapse|
|Vettel overcuts Raikkonen to end Ferrari´s Monaco wait|
|Lions 2017: Warren Gatland´s backs in profile|
|Lions 2017: Warren Gatland´s forwards in profile|
|Emery never feared the sack at PSG|
|Lions 2017: Memorable moments from tours gone by|
|Kvitova delights in ´special´ comeback win|
|I feel great - Rejuvenated Wawrinka confident ahead of French Open|
|Lions 2017: Key battles that could decide the series|
|Kerber makes unwanted history with immediate Roland Garros exit|
|Barcelona future not in my hands – Paco Alcacer|
|Varane wants to see Mbappe alongside history-maker Ronaldo|
|Iniesta: I needed Busquets´ help to lift the Copa del Rey trophy!|
|Brook facing surgery after second eye socket fracture|
|Lions 2017: The strengths and weaknesses of Gatland and Hansen|
|Man United capable of anything - Lingard eyes glory next season|
|Kvitova victorious in hugely impressive comeback|
|Freiburg´s Grifo signs Borussia Monchengladbach deal|
|Verratti expects to be at PSG next season amid Barcelona rumours|
|Coquelin: FA Cup win was for Wenger and we want him to stay|
|Raiders hold off Roosters with Rapana at the double|
|Blair replaces Bromwich as New Zealand captain|
|Ibrahimovic wanted by ´many clubs´, but he´ll stay in Europe - Raiola|
|Matuidi unsure about PSG future amid Man United links|
|Al-Khelaifi certain Emery will stay at PSG|
|Johnson stars for Red Sox, Strasburg impressive|
|Love says it´s funny to consider Cavaliers underdogs|
|MLS Review: Galaxy win Clasico, Sporting KC beaten|