Valentino Rossi topped the opening MotoGP practice at the newly resurfaced Catalunya circuit ahead of 2017 race winner Andrea Dovizioso.
The Circuit de Barcelona-Catalunya has been completely resurfaced for this year, and the original final corner has been reinstated after modifications were made to the run-off area at the turn.
Marquez set a hot early pace aboard his Honda, with the championship leader shooting to the top of the pile with a 1:40.812s, which he improved to 1:40.166s to go almost a second clear of next-best rider Rossi on the Yamaha.
Marquez lowered the benchmark further with a 1:39.882s, though future team-mate Jorge Lorenzo – who will join Honda next year after signing a two-year deal – cut his advantage down to just under half a second.
Lorenzo cut into Marquez's time gap further with a 1:40.067s with 20 minutes remaining, before Rossi split the pair in the closing stages as he started a run on fresh soft slicks.
Rossi went on to post a 1:39.456s to go fastest, while a late effort from Dovizioso on his Ducati – fitted with a fresh hard rear tyre – elevated him to second on the timesheets, though some four tenths adrift of Rossi.
Lorenzo held onto third with his final lap of 1:39.804s to miss out on his team-mate's best by just 0.001 seconds, with Marquez finishing the session fourth with his earlier gambit of 1:39.882s.
Johann Zarco completed the top five on his Tech3 YZR-M1 ahead of factory counterpart Maverick Vinales, with Cal Crutchlow on the LCR Honda a few hundredths further adrift in seventh.
Tito Rabat leaped up into the top 10 in the closing stages in eighth, with Aprilia-bound Andrea Iannone and Pramac's Jack Miller despite crashes for the pair at Turns 10 and 5 respectively.
Dani Pedrosa, who will depart Honda at the end of the year, was a low-key 12th in the opening session, splitting Aleix Espargaro on the Aprilia and the second Tech3 bike of Hafizh Syahrin.
Mika Kallio makes his second wildcard appearance of the season with KTM this weekend, and was 15th on the RC16. Sylvain Guintoli is also present for Suzuki, and was 25th in his first MotoGP session since last year's Catalan weekend.
As well as Miller and Iannone, Karel Abraham [26th] and Franco Morbidelli [23rd] also crashed during the session, both of whom going down at Turn 2.
