Starting from pole, Hamilton had dominated proceedings up until lap 64, when the safety car was deployed following a heavy shunt for Max Verstappen at Ste Devote. With the world champion leading by around 25s, Mercedes decided there was enough time for a precautionary extra pit stop.
They were wrong. Hamilton emerged from the pit lane marginally behind Vettel, with Rosberg already heading the queue behind the safety car. When racing resumed, Rosberg cleared off into the distance, while Hamilton could do nothing to overhaul the Ferrari, despite his fresher tyres.
Daniil Kvyat and Daniel Ricciardo came home fourth and fifth respectively for Red Bull’s best result of the season, followed by the second Ferrari of Kimi Raikkonen and the Force India of Sergio Perez.
Eighth place went to Jenson Button, securing the first points of McLaren’s revived Honda partnership, with Sauber’s Felipe Nasr ninth and Toro Rosso’s Carlos Sainz - who started from the pit lane - completing the top ten.
Hamilton had the race all but gift wrapped with a 25.7s advantage when Verstappen’s sterling efforts to pass Romain Grosjean ended in tears in the Ste Devote wall on the 64th lap, when he got caught out as the Frenchman braked.
At first the virtual safety car was deployed, then the real safety car. And for reasons only they know, Mercedes called Hamilton in from his impregnable position as race leader for a set of fresh supersoft tyres. Hamilton later said he complied, assuming that Rosberg and the German’s race-long shadow Vettel had already done so. They hadn’t. Mercedes left Rosberg out, and of course Ferrari did likewise with Vettel. Overtaking is always difficult here, so why stop at that late stage?
By the time Hamilton rejoined, Rosberg had overtaken him, and though Hamilton ran up the hill side by side with Vettel, replays confirmed that the German had just been in second place.
All the frontrunners stayed behind the safety car as lapped runners were allowed to un-lap themselves, then the leaders were finally allowed to get on with their race on the 71st lap. And as Vettel struggled on his harder compound tyres and held Hamilton up, Rosberg escaped.
In the end, Hamilton was unable to do anything about Vettel, and to his utter chagrin he saw not only the team mate he had dominated handed the race thanks to the highly questionable decision-making of his own team, but Vettel finish second. His championship lead is now reduced to just 10 points over Rosberg.
The question everyone was left asking was why did Mercedes surrender Hamilton’s lead for a stop that seemed so unnecessary.
And one can only imagine what others will be directed to Mercedes by Hamilton later this evening. Team boss Toto Wolff described it as the worst day of his life, and offered a public apology to his driver.
Behind the leaders, Daniel Ricciardo muscled his was past an angered Kimi Raikkonen for fifth place on the 72nd lap (contact was made in what the stewards decided was a racing incident), but after passing Red Bull team mate Daniil Kvyat he had to break off the fight with Vettel and Hamilton over second place on the final lap when he was ordered to give fourth back to his Russian team mate. Raikkonen held on for sixth, having earlier seemed deserving of fourth.
Perez brought Force India a welcome seventh, but even more welcome were Jenson Button’s first four points for the latest McLaren-Honda alliance after taking a competitive eighth on a day when team mate Fernando Alonso got a five-second penalty for punting Force India’s Nico Hulkenberg into the wall at Mirabeau on the opening lap, and then retired with gearbox trouble.
Felipe Nasr drove his heart out to take two unexpected points for Sauber, as Carlos Sainz, who started from the pit lane, kept things together to snatch tenth for Toro Rosso. Verstappen, of course, failed to finish, but while it lasted his was one of the drives of the race. He was uninjured in his accident, but was subsequently deemed to be responsible and picked up a grid penalty as a result.
Grosjean spun after the contact with him, and thus lost eighth place, so was understandably unhappy with 12th for Lotus behind Hulkenberg, who staged a strong recovery after a first-lap stop for a new nose thanks to Alonso. Marcus Ericsson brought his Sauber home 13th, as Williams’ disastrous race brought them only 14th for Valtteri Bottas and 15th for Felipe Massa, who also stopped for a new nose after contact on the first lap.
Robert Merhi finally beat Marussia team mate Will Stevens for the first time as they finished 16th and 17th respectively, one year after Jules Bianchi scored the team’s first crucial two points in ninth.
Besides Verstappen and Alonso, Lotus’s Pastor Maldonado was the first retirement with brake problems.
In the drivers’ championship battle Hamilton still leads, with 126 to Rosberg’s 116 and Vettel’s 98. In the constructors’ Mercedes still lead with 242 points to Ferrari’s 158, while Red Bull’s 22-point haul brings them up to 74, not far behind Williams’ 81.