Highlights - Monaco GP second practice (UK users only)
Fernando Alonso topped the timesheets in both of Thursday's sessions as Ferrari took the fight to Red Bull in practice for the Monaco Grand Prix.
The Spaniard's lap of one minute 14.904 seconds was 0.109secs clear of Nico Rosberg who was second in the Mercedes.
Red Bull's Sebastian Vettel set the third fastest time with Alonso's Ferrari team-mate Felipe Massa fourth.
Mercedes' Michael Schumacher was fifth, while McLaren pair Lewis Hamilton and Jenson Button came seventh and ninth.
"We weren't at the top of the times but the car felt good and I love being on this track," said Hamilton.
"We've made some good step forwards with the set-up. I think we're looking pretty good for the weekend.
"Saturday is going to be tough, [but] I hope the weather is going to be good for us and really we just have to keep our heads down and make sure we do our homework and fight for the top three in qualifying."
The impressive pace of the Red Bull cars has dominated qualifying so far this season, but Monaco's tight street circuit presents a different challenge.
The difficulties of Formula 1's slowest circuit were exacerbated by changeable weather conditions, with much of second practice session taking place under overcast skies on the edge of the Mediterranean.
Ferrari switched both of their drivers onto super soft tyres after 30 minutes of the session in order to get some running in before the expected rainfall.
That move reaped immediate dividends when Felipe Massa went quickest with a lap of one minute 15.585secs, the Brazilian then improved his own effort, before being usurped by his Ferrari team-mate.
After taking top spot Alonso stayed in the groove and produced an impressive lap of one minute 14.904secs to clock the fastest time of the day.
"Overall, we found our car very good from the beginning, very easy to drive but qualifying, everything will be very close as we saw today, it was three tenths of a second over seven or eight cars, so we can't afford any mistakes - we need to be perfect on Saturday," said Alonso.
His only anxious moment came when he touched a barrier at Casino Corner. "It was enough to break a little bit of front wing though I kept running with no problems, but you go close to the wall around here," he commented.
The changeable weather and falling track temperatures, coupled with the fact that many teams seemed to concentrate on race set up work in the afternoon, contributed to a low-key second session, which began briskly with five different drivers playing pass-the-parcel with the fastest lap time.
Williams' Nico Hulkenberg, Force India's Tonio Liuzzi, Vettel, Webber and Kubica all led the way, Kubica particularly impressive for Renault as he pushed hard to explore the limits of the famous circuit.
Kubica was seventh fastest, as only 0.716 secs separated the front-running Alonso from last week's Spanish Grand Prix winner Mark Webber, who was 10th quickest in his Red Bull.
"The other teams who have a certain top-speed advantage on other tracks at the moment do not have this advantage here, so the whole field comes a bit closer together," said Schumacher who has won at Monaco five time.
Schumacher back into rhythm of Monaco
"I got back into the rhythm of the track, especially on the long runs, and I think we can be quite confident for tomorrow as the car is definitely quite reasonable to handle," continued the 41-year-old German legend.
Several drivers voiced concern in the build-up about the potential problems that may arise from having 24 cars packed onto the tight Monte Carlo circuit and the pace differential between the teams.
There were signs of potential traffic problems, but with little at stake in practice it was difficult to gauge how much it could affect Saturday's qualifying sessions, although Force India owner Vijay Mallya later warned that it could be a "lottery".
Alonso said: "It was difficult for everybody. There were a lot of cars all together in some moments of the session. It's a challenge for the drivers to get the space to do a lap and not to disturb anyone and it's a challenge for the team to find a right time to go out.
"I think Q2 and Q3 will be much easier as the cars will be reduced. Q1 is a time to be focused, to concentrate and where you put the lap together. Maybe we don't do a super lap, maybe we just need to be in the first 15 or 16, so we should be able to do that."
Alonso's team-mate Massa was one of several drivers to struggle with overrunning into Sainte Devote and the yellow flag was briefly out when Bruno Senna went straight on at the Loews Hairpin.
Thursday's first session saw BMW Sauber's Kamui Kobayashi lose his front wing after bouncing over the kerbs at the Swimming Pool chicane and smashing into the barriers on the outside of the track.
In an earlier incident, Hispania's Karun Chandhok lost the back end of his car at Massenet but only brushed the barrier during the resultant spin, thus minimising the damage.
Highlights - Monaco GP first practice (UK users only)