Fernando Alonso's bid to win the Indy 500 at his first attempt came to a disappointing end as engine failure robbed him of a strong finish at Indianapolis Motor Speedway.
The McLaren F1 driver skipped the Monaco Grand Prix to contest the iconic oval race and led for a total of 27 laps before his Andretti Autosport Honda retired on lap 179 of 200.
Ex-F1 driver Takuma Sato came through to win the 101st staging of the race, edging out three-time victor Helio Castroneves in a breathless conclusion.
"I felt the noise and the engine friction so I backed off," said Alonso. "It's a shame because I felt we deserved to finish and experience the last lap - who knows where we could have finished."
Sato, who raced in F1 between 2002 and 2008 before switching to the American series, took the lead on lap 195 before successfully fending off the challenge of veteran Brazilian Castroneves.
Fellow former F1 driver Max Chilton of Britain also had a superb race, leading for several laps before coming home in an eventual fourth place. Dubai-born British driver Ed Jones took an impressive third place on his Indy 500 debut.
The race was earlier overshadowed by an incident involving 2008 winner Scott Dixon, who emerged largely unscathed after crashing heavily on lap 53.
Alonso experiences more Honda engine woes
Alonso's Andretti Autosport team-mate Ryan Hunter-Reay was among the favourites for the win and a huge threat to his ambitions, but the American's hopes literally went up in smoke on lap 138 when his Honda engine let go.
That left Alonso in the lead and, given his tribulations in F1 over the last couple of years, the irony of a Honda engine helping him into that position would not have been lost on the Spaniard.
A delayed pit stop for last year's winner Alexander Rossi - another from the Andretti Autosport stable - also played into Alonso's hands as the race entered the closing stages.
But things started to go awry for two-time F1 world champion Alonso when he lost a few places on the restart that followed the Hunter-Reay caution period.
As he tried to battle his way through the pack from ninth place in the closing stages it was his turn to experience the all-too-familiar sensation of a Honda engine failing at his back.
Another caution period followed - after a five-car smash - before the run to the flag finally saw Andretti Autosport driver Sato emerge in front to become the first Japanese winner of the race.
Dixon survives monster smash
The race was littered with caution periods, although the first one took 53 laps to arrive. However, when it came it was a significant one and caused the red flag to come out, stopping the race.
Dixon's Chip Ganassi Racing car was flipped high into the air and came down on its side on the infield wall after colliding with British driver Jay Howard, who was out of control and fighting a damaged car after running wide.
New Zealander Dixon escaped serious injury in the ferocious impact, which saw his Dallara chassis cleaved in two but mercifully remain intact around the driver's survival cell.
The accident capped a tumultuous few days for four-time IndyCar champion Dixon, who secured pole for the race earlier in the week but was robbed at gunpoint at a fast-food restaurant just hours later.
"I am fine - just a little beaten up there. I am just bummed for the team," Dixon said after being released with a clean bill of health from the medical centre, although he was later seen with a protective boot on his left foot.
"It was definitely a wild ride. We owe a big thanks to the safety standards we have now."
Analysis: Will the rookie return to win?
By Andrew Benson, Chief F1 writer:
Fernando Alonso's Indianapolis 500 adventure ended as have so many of his recent Formula 1 outings - with a Honda engine failure. But his trip to America was a resounding triumph for both the Spaniard and his McLaren F1 team.
Alonso was contending for victory throughout the Indy 500, had the highest average lap speed in the race, and had just pulled a beautiful outside pass around Turn One on Brazilian veteran Tony Kanaan to take sixth place as the race entered its closing laps.
He was a 'rookie', as the Americans call it, but he drove like he had been racing on ovals all his life.
In one way, it was to be expected - he is, after all, one of the greatest racing drivers in history. But the difficulty of adapting to the unique challenges of oval racing at the daunting 230mph Indianapolis Motor Speedway should not be underestimated.
The regulars were deeply impressed, and Alonso's US foray excited millions of fans around the world and unquestionably enriched his legend.
Indy is a hard race to win, but Alonso proved he was more than capable of doing it.
Now, it's back to the day job in Canada in two weeks' time. He will go there with his reputation enhanced and, one imagines, a deepened determination to return and get the job done another time.
'One of the best experiences of my career'
Fernando Alonso: "The whole thing has been a nice experience. The racing was fun and I am glad to been here with the best racers in the business.
"It's early [to decide if he comes back] but I feel competitive and if I come back I will feel like I know what to expect so will use that next time.
"Thanks to IndyCar because this has been one of the best experiences of my career."
Race winner Takuma Sato: "Unbelievable! It's the best feeling. It's beautiful. I can't thank enough this team. Hopefully the crowd enjoyed it!
"With three laps to go I really didn't know but I just knew I had to go for it!"