A late flurry of laps on a drying track surface after prolonged mid-session drizzle saw the timesheets shuffling rapidly, with Edo Mortara coming out on top.
The Venturi man set the benchmark with one of the final laps of the session, pipping reigning champion Antonio Felix da Costa (DS TECHEETAH) by 0.031 seconds whilst Oliver Turvey hoisted his NIO 333 and its all-new powertrain into third.
Alexander Sims looks to have made himself right at home at his new team, Mahindra Racing, on this first morning back in the office, whilst Sergio Sette Camara (DRAGON / PENSKE) and Nick Cassidy (Envision Virgin Racing) rounded out the top six runners.
READ MORE: TESTING: Saturday updates from Valencia
Mortara's 1:18.148 was partly a byproduct of that drying track after steady drizzle soaked the surface throughout the middle third of the morning session, with 18 cars separated by just 0.7 seconds come the chequered flag.
Those 18 runners all lapped beneath the 1:19 mark, with early the first hour's running a second slower still as Circuit Ricardo Tormo shook off the standing water after yesterday's torrential rain and thunderstorms.
All 12 teams managed good mileage from both of their drivers with little drama, save for brief excursions into the Sector 1 gravel for Sebastien Buemi (Nissan e.dams) and Sette Camara. Everybody worked through their intricately planned practice programmes, seeking to prove their new powertrains and cars.
As always in testing, being fastest isn't always the priority, with a lot of work to be done between now and close of play on Tuesday.