Nico Rosberg set the pace in Thursday's second practice as team-mate Lewis Hamilton came to the fore, making it a Merc 1-2.
Using the supersoft Pirelli tyres for the first time in Monaco, Rosberg posted a blistering 1:14.759 to beat Hamilton by 0.318s.
Fernando Alonso was third for Ferrari, 0.437s down, while Romain Grosjean was the day's only crasher.
Unlike in FP1, the drivers were quickly into the action on Thursday afternoon as the track temperature reached a sweltering 45'C. Esteban Gutierrez posted the first lap time, a 1:24.8, but five minutes later he was down in 16th place, 19 drivers had set times, and Grosjean was quickest of all with a 1:16.096 after setting three purple sector times.
Minutes later it was a Mercedes 1-2 with Rosberg leading the way with a 1:15.843, a tenth up on his team-mate Hamilton. Grosjean was third ahead of Mark Webber and Alonso while Sebastian Vettel finally joined the action, the last driver to do so.
Fernando Alonso was the next to lead the way before he was overhauled by Kimi Raikkonen with a 1:15.511. It was the Lotus driver's second flying lap on the supersoft tyres. Meanwhile Vettel put in his first lap, going seventh quickest, half a second down.
Rosberg was the next out on the red-ringed tyres and immediately went quickest with a flying 1:14.759. That put him 0.752s up on Raikkonen! Hamilton went second, 0.318s down.
Vettel returned to the pits after just five laps with reports emerging that his RB suffered a KERS failure. Grosjean's woes, though, were worse as the Frenchman locked up through St Devote and smacked the barriers, damaging the left front of the Lotus.
Although it was initially double waved yellows, damage to a kerb at Turn 13 resulted in the session being red flagged for a minute while marshals used a crownbar to clear something they were unhappy with it.
With the action back underway others ventured out on the supersoft tyres but could not challenge Rosberg's 1:14.7. Even his team-mate Hamilton couldn't match the German's best.
As has become the story of this season, the drivers spent the last 30 minutes running on old tyres with heavy fuel loads meaning there were no changes to the timesheets.
As such the day ended with Rosberg quickest with a 1:14.759 and Hamilton second.
01 Nico Rosberg Mercedes 1:14.759 45 laps
02 Lewis Hamilton Mercedes 1:15.077 +0.318s 50 laps
03 Fernando Alonso Ferrari 1:15.196 +0.437s 37 laps
04 Felipe Massa Ferrari 1:15.278 +0.519s 38 laps
05 Mark Webber Red Bull 1:15.404 +0.645s 41 laps
06 Kimi Raikkonen Lotus 1:5.511 +0.752s 38 laps
07 Romain Grosjean Lotus 1:15.718 +0.959s 10 laps
08 Jenson Button McLaren 1:15.959 +1.200s 39 laps
09 Sebastian Vettel Red Bull 1:16.014 +1.255s 32 laps
10 Paul di Resta Force India 1:16.046 +1.287s 42 laps
11 Adrian Sutil Force India 1:16.349 +1.590s 43 laps
12 Sergio Perez McLaren 1:16.434 +1.675s 40 laps
13 Nico Hulkenberg Sauber 1:16.823 +2.064s 42 laps
14 Pastor Maldonado Williams 1:16.857 +2.098s 40 laps
15 Esteban Gutierrez Sauber 1:16.935 +2.176s 44 laps
16 Daniel Ricciardo Toro Rosso 1:17.145 +2.386s 37 laps
17 Jean-Eric Vergne Toro Rosso 1:17.184 +2.425s 42 laps
18 Valtteri Bottas Williams 1:17.264 +2.505s 46 laps
19 Jules Bianchi Marussia 1:17.892 +3.133s 40 laps
20 Charles Pic Caterham 1:18.212 +3.453s 43 laps
21 Max Chilton Marussia 1:18.784 +4.025s 40 laps
22 Giedo van der Garde Caterham 1:19.031 +4.272s 30 laps
Thursday's Pics: 1 Dresses Up For Monaco