By Alan Baldwin
MONACO, May 21 (Reuters) - Formula One world champion Lewis Hamilton was fastest but teenage rookie Max Verstappen stood out with an eye-catching performance in a damp opening practice for the Monaco Grand Prix on Thursday.
Hamilton, fresh from signing a new three-year deal with Mercedes, lapped the tight street circuit with a best time of one minute 18.750 seconds on a cool and overcast morning.
While that was no great surprise from the championship leader, who is out to deny team mate Nico Rosberg a third win in a row in the Mediterranean principality, Verstappen's performance was highly impressive.
The 17-year-old, the only driver in the field never to have raced before in Monaco, looked a natural as he lapped only 0.149 slower in his Toro Rosso after completing 42 near-faultless laps.
The Dutch driver, son of former racer Jos, popped in his fastest lap just before the chequered flag when the track had most grip.
Rosberg, 20 points behind Hamilton after five of 19 races this season, was ninth fastest in a session that started with drivers on intermediate tyres after a heavy overnight storm.
With the track drying, they swiftly switched to slick tyres on the slippery asphalt.
Rosberg brushed the barriers without consequences at Tabac and several others went down the escape road at the Ste Devote corner.
Australian Daniel Ricciardo ended the session third fastest for Red Bull, ahead of Ferrari's Sebastian Vettel and Toro Rosso's Spanish rookie Carlos Sainz.
Britain's Jenson Button sat out the first hour while McLaren mechanics worked on his car but eventually surfaced to set the 12th best time with team mate Alonso 11th. (Editing by John O'Brien)