Lewis Hamilton, aiming for a fifth straight win of the season at the Monaco Grand Prix, edged his Mercedes teammate Nico Rosberg in Thursday's opening free practice session.
The 29-year-old Briton clocked a best lap of one minute 18.271 seconds around the tight and unforgiving barrier-lined Mediterranean street circuit, with his friend and closest rival just 0.032 seconds behind.
They each topped the times in the contest before Hamilton, the 2008 champion and championship leader, asserted his position with the best lap midway through the 90-minute session.
Australian Daniel Ricciardo of Red Bull broke into their personal scrap when he briefly went fastest early on and he was one of only two other drivers to break the 1:19 barrier.
The other was fourth-placed Fernando Alonso for Ferrari.
This left defending four-time drivers' world champion Sebastian Vettel down in fifth on a blustery, shower-strewn and cool day in the Principality.
Like Vettel, Alonso's Ferrari team-mate Kimi Raikkonen, the 2007 champion, improved in the final minuters to take sixth ahead of compatriot Valtteri Bottas, in the leading Williams.