Lewis Hamilton emerged quickest in first practice for the Monaco Grand Prix as he resumed his duel with Mercedes Formula 1 team-mate Nico Rosberg.
With Pirelli's ultra-soft tyre making its race debut, Hamilton set the pace with a lap of 1m15.537s, just half-a-second slower than last year's pole position time.
The session was red-flagged with three minutes to go as it emerged a drain cover had worked loose near Ste Devote, causing punctures to the left-rear of Rosberg's car and the front-right of Jenson Button's McLaren after hitting debris.
Early running on the ultra-softs from the Mercedes duo had Hamilton setting the early benchmark at 1m16.588s, two seconds better off than the corresponding session last year.
Just 24 minutes in, Felipe Massa lost control of his Williams coming out of Ste Devote, severely damaging the left front and rear of his car as he clouted the barrier, resulting in a virtual safety car period.
Midway through the session Hamilton and Rosberg again traded quickest laps, with the former again taking the honours, conjuring a time that would remain unbeaten.
It is anticipated that in dry conditions the pole time will be two seconds faster than in 2015, around the 1m13s mark.
Shortly after, though, Hamilton had his own moment at Ste Devote, carrying too much speed into the first corner and locking up into the escape road, albeit comfortably short of the barrier.
Hamilton was not the only driver to make a mistake at that turn as numerous others followed suit.
Hamilton ultimately finished a tenth of a second quicker than Rosberg, with Ferrari's Sebastian Vettel next up and 0.419 seconds adrift.
The leading trio were then followed by Red Bull pair Daniel Ricciardo and Max Verstappen, with Ricciardo enjoying the benefit of the new Renault engine, along with Kevin Magnussen.
The laptime gain for Monaco is anticipated to be 0.2s, with Ricciardo's time on this occasion, though, just 0.063s better off than Verstappen.
Sixth-placed Daniil Kvyat in his Toro Rosso was the only other driver to finish within a second of Hamilton.
Kimi Raikkonen was only ninth quickest in his Ferrari, with the Finn 1.375s off the pace.
Renault's Jolyon Palmer also brought up the virtual safety car just after the hour mark, crashing into the barriers at Tabac, finishing 20th and over three seconds down, ahead of only Manor pair Rio Haryanto and Pascal Wehrlein.
FIRST PRACTICE RESULTS:
|4||Daniel Ricciardo||Red Bull/Renault||1m16.308s||0.771s||29|
|5||Max Verstappen||Red Bull/Renault||1m16.371s||0.834s||30|
|6||Daniil Kvyat||Toro Rosso/Ferrari||1m16.426s||0.889s||37|
|7||Nico Hulkenberg||Force India/Mercedes||1m16.560s||1.023s||34|
|8||Sergio Perez||Force India/Mercedes||1m16.697s||1.160s||28|
|10||Carlos Sainz Jr.||Toro Rosso/Ferrari||1m17.130s||1.593s||39|