This year's European Formula 3 champion hit the front in the evening session of what was his first outing in GP2, setting the outright pace with a 1m49.811s.
Only two drivers - series regular Alexander Rossi and Red Bull junior Carlos Sainz - were able to join him in the sub 1m50s bracket.
The former set the pace for Caterham in the morning session with a 1m49.958s, leaving him more than half a second clear of a chasing pack led by Daniel Abt, Johnny Cecotto and last year's GP3 champion Mitch Evans, who was testing for newly-crowned teams' champions Russian Time.
Rossi's time was ultimately enough to earn him second overall, although Carlin's Sainz ended the evening just nine thousandths of a second down the road.
Sainz's time had kept him top of the evening pile until the final hour, when Marciello improved to end the day at the head of the timesheets.
With the exception of Rossi, all top 10 times were set in that latter session.
Abt - testing for Hilmer - secured fourth overall with a 1m50.120s.
Less than one tenth of a second, meanwhile, covered Cecotto (Lazarus), GP2 race-winner Jolyon Palmer (Racing Engineering), Evans and Arden's Rene Binder.
Will Stevens, who took five podiums in Formula Renault 3.5 this year, secured ninth for MP Motorsport, fractions ahead of fellow FR3.5 podium sitter Andre Negrao, driving for Hilmer.
Among a number of changes for day two are Palmer switching to DAMS, British F3 champion Jordan King coming in at Addax and Sam Bird substituting for Tom Dillmann at Russian Time, the team with which he finished runner-up in GP2 this year.
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- Motor Racing
- Alexander Rossi
- Johnny Cecotto
- Mitch Evans
- Daniel Abt