Harry Tincknell and Jonathan Bomarito clinched Mazda team Joest's third consecutive IMSA SportsCar Chmapionship win, despite a final-lap error threatening to derail their Road America victory bid.
The #55 Mazda TR24-P duo beat the championship-leading Penske Acura ARX-05 of Dane Cameron and Juan Pablo Montoya, winning for the second time in 2019 after clinching the squad's first IMSA victory at Watkins Glen in June.
The two Acuras of Cameron and Helio Castroneves eased into an early one-two, chased by the Mazdas of Oliver Jarvis and Bomarito, as the Action Express Racing and Wayne Taylor Racing Cadillac DPi-V.Rs of Pipo Derani and Renger can der Zande respectively hung on to the lead pack.
Bomarito first made ground by passing the sister #77 car of Jarvis on lap 11, three laps before Castroneves pitted from second - a move that allowed him to jump the #7 Acura ahead of the sister #6 car, which Cameron handed over to Montoya at its first stop.
The #55 Mazda closed in on the leading Acuras once Bomarito had served his first pitstop, and just a second covered the top three when the all pitted on lap 30 - with Castroneves handing over to Ricky Taylor, Montoya staying in the #6 Acura and Tincknell taking Bomarito's place.
Taylor was forced to visit the pits again just four laps later after picking up debris from a GT Daytona car, which put Montoya - struggling to hold off Tincknell - into the lead.
Tincknell then grabbed first when Montoya lost momentum behind an LMP2 runner, slipstreaming past on the main straight and into the first corner, before extending a lead of 3.5 seconds.
That was slashed when Tincknell drifted onto the grass at Turn 3 after lapping a GT Daytona car, but by the time the two leaders had pitted again - Montoya to hand back over to Cameron - the #6 was eight seconds off the lead, and coming under pressure Jarvis, back in the #77 in place of Tristan Nunez.
The second- and third-place cars had got back within four seconds of Tincknell with less than an hour left on the clock, largely thanks to traffic, but both Cameron and Jarvis then lost time behind the JDC Miller Cadillac before their final stops.
Both stopped with half an hour remaining, staying in the same order, with Jarvis rejoining slightly further adrift of Cameron and then losing more time when Stephen Simpson (JDC Miller) dived past him at Turn 6 to unlap himself.
Tincknell had rejoined from him own final stop by this point and held a comfortable four-second advantage over Cameron, but in the closing laps he lost time in traffic - allowing the Acura to suddenly close up.
The leading Mazda then ran wide at Turn 4 on the final lap, allowing Cameron to start nosing alongside, but Tincknell squeezed the Acura hard with slight contact and resisted the challenge - winning by 0.227s.
Jarvis ended up 2.9s adrift in third, with the leading trio more than 70s clear of the top Cadillac DPi-V.R of Felipe Nasr and Derani in fourth, which finished four seconds ahead of the Wayne Taylor Racing entry.
Matt McMurray and Patrick Kelly were best of the LMP2s in 10th, four laps down, with their PR1/Mathiasen Motorsports ORECA beating the similar Performance Tech Motorsports car of Cameron Cassels and James French by half a minute.
Ryan Briscoe and Richard Westbrook secured GTLM honours in the #67 Ford GT, Westbrook beating Joey Hand in the sister #66 car he shared with Dirk Muller by 10.657s.
Earl Bamber brought the #912 Porsche home a further 17s behind, ahead of Corvette driver Antonio Garcia - who displaced Connor de Phillippi's BMW M8 at the last gasp.
Pfaff Motorsports' Porsche 911 GT3 R ran out victories in GT Daytona, as Matt Campbell held on to beat the Paul Miller Lamborghini Huracan of Bryan Sellers.
Result - 83 laps
|1||DPi||J.Bomarito, H.Tincknell||Mazda Team Joest||Mazda||2h41m43.115s|
|2||DPi||J.P.Montoya, D.Cameron||Acura Team Penske||Acura||0.227s|
|3||DPi||O.Jarvis, T.Nunez||Mazda Team Joest||Mazda||3.114s|
|4||DPi||F.Nasr, P.Derani||Whelen Engineering Racing||Cadillac||1m16.250s|
|5||DPi||R.van der Zande, J.Taylor||Konica Minolta Cadillac DPi-V.R||Cadillac||1m20.362s|
|6||DPi||J.Barbosa, F.Albuquerque||Mustang Sampling Racing||Cadillac||1m50.473s|
|7||DPi||R.Taylor, H.Castroneves||Acura Team Penske||Acura||1m55.838s|
|8||DPi||M.Goikhberg, T.Vautier||JDC-Miller Motorsports||Cadillac||1 Lap|
|9||DPi||S.Trummer, S.Simpson||JDC-Miller Motorsports||Cadillac||1 Lap|
|10||LMP2||M.McMurry, P.Kelly||PR1/Mathiasen Motorsports||ORECA/Gibson||4 Laps|
|11||LMP2||C.Cassels, J.French||Performance Tech Motorsports||ORECA/Gibson||4 Laps|
|12||GTLM||R.Briscoe, R.Westbrook||Ford Chip Ganassi Racing||Ford||6 Laps|
|13||GTLM||J.Hand, D.Muller||Ford Chip Ganassi Racing||Ford||6 Laps|
|14||GTLM||E.Bamber, L.Vanthoor||Porsche GT Team||Porsche||6 Laps|
|15||GTLM||J.Magnussen, A.Garcia||Corvette Racing||Chevrolet||6 Laps|
|16||GTLM||T.Blomqvist, C.De Phillippi||BMW Team RLL||BMW||6 Laps|
|17||GTLM||O.Gavin, T.Milner||Corvette Racing||Chevrolet||7 Laps|
|18||GTLM||P.Pilet, N.Tandy||Porsche GT Team||Porsche||7 Laps|
|19||GTD||M.Campbell, Z.Robichon||Pfaff Motorsports||Porsche||8 Laps|
|20||GTD||B.Sellers, C.Lewis||Paul Miller Racing||Lamborghini||9 Laps|
|21||GTD||B.Auberlen, R.Foley||Turner Motorsport||BMW||9 Laps|
|22||GTD||R.Heistand, J.Hawksworth||AIM Vasser Sullivan||Lexus||9 Laps|
|23||GTD||M.Farnbacher, T.Hindman||Michael Shank Racing with Curb-Agajanian||Acura||9 Laps|
|24||GTD||J.Potter, A.Lally||Magnus Racing||Lamborghini||9 Laps|
|25||GTD||C.MacNeil, T.Vilander||Scuderia Corsa||Ferrari||9 Laps|
|26||GTD||A.Imperato, D.Olsen||Wright Motorsports||Porsche||9 Laps|
|27||GTD||F.Montecalvo, T.Bell||AIM Vasser Sullivan||Lexus||9 Laps|
|28||GTD||P.Chase, R.Dalziel||Starworks Motorsport||Audi||9 Laps|
|29||GTD||G.Robinson, L.Aschenbach||Lone Star Racing||Mercedes||9 Laps|
|30||GTD||P.Holton, M.Plumb||Compass Racing||McLaren||10 Laps|
|31||GTD||K.Legge, A.Beatriz||Heinricher Racing with Meyer Shank Racing||Acura||11 Laps|
|32||GTLM||J.Krohn, J.M.Edwards||BMW Team RLL||BMW||11 Laps|
|33||GTD||B.Keating, J.Bleekemolen||Mercedes-AMG Team Riley Motorsports||Mercedes||15 Laps|
|34||DPi||J.Bennett, C.Braun||CORE Autosport||Nissan||Not running|
|35||GTD||P.Lindsey, P.Long||Park Place Motorsports||Porsche||Not running|