Pastor Maldonado appeared to lose his cool in final practice for tomorrow's Monaco Grand Prix.

Just 13 days ago Maldonado was a hero for Williams in winning his debut race and the team's first for seven-and-a-half years by taking the chequered flag in Barcelona.

But towards the end of the hour-long session around the streets of the principality, the Venezuelan first appeared to turn into Sauber's Sergio Perez before later crashing into a barrier that brought the session to a halt four minutes early.

Replays caught Maldonado clashing with Perez going through Portier - the corner before the entry to the tunnel - appearing to deliberately cut across the Mexican who reported the incident to his team over the in-car radio, claiming: "Maldonado is crazy."

On the following lap, and with potential damage to his car, the 26-year-old Maldonado then hit the barrier on entry into Casino Square.

The impact sent him colliding into the barrier on the opposite side of the circuit, resulting in him losing his left-rear wheel as he then trundled down the short straight to Mirabeau.

It immediately led to the session being red-flagged, and with so little time remaining there was no restart, leaving Maldonado - a two-time winner here in junior categories - down in 21st.

It remained to be seen whether Maldonado would be hauled before the stewards, and also whether his team would have time to repair the car before qualifying.

Come the conclusion, Mercedes' Nico Rosberg led the way with a lap of one minute and 15.159 seconds, with the top four drivers covered by just five hundredths of a second such was its close nature.

For the first time this year Ferrari's Felipe Massa appears to be on the pace as he finished just 0.038secs down, followed closely by reigning champion Sebastian Vettel in his Red Bull and the second Ferrari of team-mate Fernando Alonso.

Lotus' Romain Grosjean was 0.286secs adrift in fifth, with the McLarens of Jenson Button and Lewis Hamilton sixth and seventh, the latter just over half a second off the pace.

Michael Schumacher in his Mercedes, Perez and Red Bull's Mark Webber completed the top 10.

Force India's Paul di Resta was also in the wars late on, the Scot riding a bump down the hill out of the tunnel and heading towards the port chicane, forcing him nose first into a barrier.

Di Resta's damage was minimal compared to that of Maldonado, but it was still enough to bring his session to a halt six minutes early.

The HRT of Pedro de la Rosa brought up the rear of the field with a time nearly four seconds down on Rosberg.