Monaco – Lewis Hamilton lapped faster than ever at Monaco as the Mercedes driver and Ferrari's championship leader Sebastian Vettel set the pace in first practice for Formula One's showcase race on Thursday.
The triple world champion, who can equal the 65 career pole positions of boyhood idol Ayrton Senna in Saturday's qualifying, topped the timesheets with a best lap of one minute 13.425 seconds on ultrasoft tyres.
Vettel, a four times world champion with Red Bull who leads Hamilton by six points after five races, was second and a mere 0.196 slower than the Briton with a time of 1:13.621.
Both men were inside Australian Red Bull driver Daniel Ricciardo's 2016 pole position time of 1:13.622 with Hamilton's the fastest yet around the metal-fenced harbourside street circuit.
Hamilton did 40 laps, Vettel 34.
The Ferrari driver is hoping to end his team's long drought in the most glamorous – and slowest – race on the calendar with a first Ferrari win in the Mediterranean principality since Michael Schumacher in 2001.
Under cloudy skies, after Wednesday's bright sunshine, Britain's Jenson Button returned to the cockpit as a McLaren stand-in for Fernando Alonso, who is competing at the Indianapolis 500.
The 2009 champion was 14th fastest, 1.529 off the pace, and completed 35 trouble-free laps in a solid performance for a driver who has not been in a Formula One car since last November.
Red Bull's Max Verstappen was third quickest, a positive development for the former champions, ahead of Hamilton's Finnish team mate Valtteri Bottas and Ricciardo in fifth.
Renault's Nico Hulkenberg and Sauber's Marcus Ericsson both failed to set a time, with the latter suffering a gearbox problem.
Practice in Monaco is always on a Thursday due to Friday, the usual day for first running on track at other races, being a rest day.
Monaco Grand Prix: First Free Practice
1. Lewis Hamilton (Britain) Mercedes 1:13.425
2. Sebastian Vettel (Germany) Ferrari 1:13.621
3. Max Verstappen (Netherlands) Red Bull – TAG Heuer 1:13.771
4. Valtteri Bottas (Finland) Mercedes 1:13.791
5. Daniel Ricciardo (Australia) Red Bull – TAG Heuer 1:13.854
6. Daniil Kvyat (Russia) – Renault 1:14.111
7. Kimi Raikkonen (Finland) Ferrari 1:14.164
8. Sergio Perez (Mexico) Force India – Mercedes 1:14.201
9. Carlos Sainz Jr (Spain) Toro Rosso – Renault 1:14.333
10. Esteban Ocon (France) Force India – Mercedes 1:14.425
11. Felipe Massa (Brazil) Williams-Mercedes 1:14.617
12. Stoffel Vandoorne (Belgium) McLaren 1:14.813
13. Kevin Magnussen (Denmark) Haas – Ferrari 1:14.870
14. Jenson Button (Britain) McLaren 1:14.954
15. Romain Grosjean (France) Haas – Ferrari 1:15.321
16. Lance Stroll (Canada) Williams-Mercedes 1:15.595
17. Jolyon Palmer (Britain) Renault 1:15.949
18. Pascal Wehrlein (Germany) Sauber – Ferrari 1:16.258
19. Nico Huelkenberg (Germany) Renault
20. Marcus Ericsson (Sweden) Sauber – Ferrari