Rosberg set the initial pace, but his 1m 18.477s was beaten by Hamilton with 1m 18.271s and that remained the best time. At that stage, Ricciardo was 1.458s adrift, but the Australian was driving with maximum confidence in his RB10 and carved down from 1m 19.729s to 1m 19.047s, and then to 1m 18.506s. Two subsequent efforts to improve on that fell short. Rosberg also improved, but 1m 18.303s failed to dislodge Hamilton.
By the time Ricciardo moved into his challenging position, however, Mercedes had already started to focus on long-distance runs, after a revised weather forecast suggested a possibility of rain in FP2.
Behind them Fernando Alonso, at his most dangerous with half a chance and his back to the wall, sliced down to 1m 18.930s for fourth. Sebastian Vettel also got within one second of Hamilton, managing 1m 19.043s for fifth ahead of Kimi Raikkonen on 1m 19.467s in the second Ferrari.
Williams' Valtteri Bottas was seventh on 1m 19.494s, Force India's Sergio Perez eighth on 1m 19.666s and McLaren's Kevin Magnussen ninth on 1m 19.789s. Perez's team mate Nico Hulkenberg was 10th on 1m 19.856s, as Jenson Button took 11th for McLaren in 1m 20.033s.
Esteban Gutierrez was an encouraging 12th for Sauber on 1m 20.118s from the Lotuses of Romain Grosjean and Pastor Maldonado on 1m 20.207s and 1m 20.241s respectively, with Jean-Eric Vergne on 1m 20.260s for Toro Rosso from Williams' Felipe Massa on 1m 20.517s and Sauber's Adrian Sutil on 1m 20.736s.
Daniil Kvyat meanwhile underlined his talent by managing to set a time - 1m 20.914s - that was only seven-tenths off his experienced team mate Vergne on what was his first-ever run at Monaco, leaving the Russian in 18th.
Jules Bianchi's 1m 21.310s initially stood him 12th until others improved, but the Marussia driver still led the newbie teams. Marcus Ericsson did a strong job with Caterham to get within half a second with 1m 22.063s. Their respective team mates were 22nd and 21st, with Max Chilton on 1m 25.817s and Kamui Kobayashi on 1m 22.492s.
Surprisingly, given the misgivings several drivers had expressed about the likely slipperiness of the new asphalt from Casino Square down to the tunnel, there were only two major incidents during the session, and both happened at Mirabeau. Chilton spun his Marussia there early on and took no further part, while Sutil lost the back end of his Sauber there in the final 15 minutes and clobbered the wall hard enough to damage the left rear suspension.
Right at the end of the session Alonso locked his left-front tyre and went straight on at Mirabeau - crucially without making contact with anything - while Gutierrez did likewise at St Devote.