In a dramatic moment the drain cover was flicked back into the path of Jenson Button's McLaren, causing substantial damage to the Briton's front wing and leading to the session being red flagged with three minutes to go.
Williams and Renault were also in the wars. At the former, Felipe Massa got it wrong coming into Ste Devote less than half an hour in, and badly damaged the left-side of his car against the barriers.
And in the final half hour Jolyon Palmer made an error of judgement at Tabac and similarly left his Renault mechanics with plenty of repair work to do ahead of FP2.
Closest to the Mercedes in the times was Ferrari’s Sebastian Vettel, followed by the Red Bulls of Daniel Ricciardo and Max Verstappen. Daniil Kvyat was sixth for Toro Rosso, ahead of the Force India duo of Nico Hulkenberg and Sergio Perez.
Ferrari’s Kimi Raikkonen and Toro Rosso’s Carlos Sainz completed the top ten.
For the most part, Hamilton ruled the 90 minutes, apart from brief moments when Rosberg edged ahead. The world champion set the pace initially with 1m 16.588s for Mercedes, as Ricciardo, Verstappen and Kvyat filled out positions two to four, each with something to prove.
Hamilton later improved to 1m 16.057s on Pirelli’s ultrasoft tyre before Rosberg went ahead with 1m 15.825s, and the Englishman’s 1m 15.537s response thereafter remained fastest until the end.
As Rosberg trimmed his lap to 1m 15.638s, Vettel jumped to third, also on the purple-marked rubber, with 1m 15.956s after a quiet start for Ferrari.
Kvyat was the highest placed driver that didn’t use the ultrasofts, though the Russian was also one of several drivers - including Hamilton and Sauber’s Felipe Nasr - to run up the escape road at Turn 1.
Haas were another team to have a mixed morning, with Esteban Gutierrez rolling to a halt with engine issues on the run down to Portier. The Mexican had earlier got away with a big moment braking for the chicane, as did Renault’s Kevin Magnussen, while Williams’ Valtteri Bottas was lucky to escape the barriers after getting sideways out of the tunnel.
It all bodes well for another cracking session this afternoon.