Nico Rosberg moved to within two points of Lewis Hamilton in the title race after taking advantage of the world champion's terrible start to win the Italian Grand Prix.
Hamilton was widely expected to capture his 50th career victory and third in succession at Monza after dominating in qualifying on Saturday.
But a woeful getaway saw the British driver fall to sixth by turn one, and while a one-stop strategy enabled him to leapfrog the Ferrari duo of Sebastian Vettel and Kimi Raikkonen to finish second, it was Rosberg who cruised to the chequered flag with Vettel completing the podium.
Hamilton, 31, was bidding to become the first driver since the great Juan Manuel Fangio in the 1950s to claim a hat-trick of consecutive victories at the so-called Temple of Speed.
But a poor start, which cost Hamilton probable victories at the opening two rounds in Australia and Bahrain, returned to haunt him here on Sunday.
Gobbled up by Rosberg, both Ferrari drivers and then the Williams of Valtteri Bottas and Red Bull's Daniel Ricciardo on the long run down to the opening chicane, Hamilton was left facing an almost impossible task of winning for a third year on the trot in Italy.
On the second lap Hamilton passed Ricciardo with a sleek move at the second chicane, but he struggled to make his way past Bottas. Indeed by the time Hamilton eventually took the Finn on lap 11 he was already 11 seconds behind Rosberg.
Vettel and Raikkonen were running line astern in second and third, but their decision to stop for tyres on two occasions enabled Hamilton to leapfrog the Ferrari pair.
Unlike here two years ago when Rosberg made two separate mistakes at the first chicane to gift-wrap victory to Hamilton, an error-free performance from the German saw him secure his second lights-to-flag win in as many weekends, and his seventh of the season.
Suddenly the momentum which has slowly been slipping away from Rosberg after he won the opening four races of the year is falling back in his favour.
Hamilton finished 15 seconds down on his Mercedes team-mate.
Jenson Button will not be racing here in 2017 after he announced he is stepping down from first-team duties for McLaren at the end of this season, and following another middling display here, in which he crossed the line 12th, the Englishman will probably be looking forward to the break.
Indeed Hamilton could be the only Briton on the grid next term with Jolyon Palmer's bid to secure a second campaign in the sport not helped by an early retirement following his crash with Felipe Nasr in the opening stages. The Sauber driver was deemed at fault for the incident and slapped with a timed penalty.
Elsewhere, in a rather dreary race, Ricciardo executed a fine move in the closing stages to take fifth from Bottas while Max Verstappen also squeezed past Sergio Perez in the latter moments as they finished seventh and eighth respectively. Fernando Alonso was a lowly 14th.