The day was broken up by numerous heavy rain showers as the Yamaha Factory Racing rider led Repsol Honda Team duo Dani Pedrosa and Marc Márquez.
The first half of the day was all but a complete washout. As the session began, sunshine and dark clouds were vying for the circuit as they each covered half of it, but it was the latter which prevailed to bring several, heavy rainstorms.
As the showers were intermittent, the field was never treated with either fully wet or dry conditions which resulted in most riders spending much time in their garages; as only four sets of Wet tyres are available for the duration of the test, using them in the transitional conditions posed too much of a threat and overheating was a real possibility. Four hours in, Pedrosa led the way but only nine runners had elected to post a lap time.
From the outset, Honda had never planned to run unless they believed it was completely meaningful, as the team was only intending to confirm information it had learned at the private test in Austin.
Others had entirely different agendas, not least Hiroshi Aoyama who was highly keen to ride his Avintia Blusens CRT bike in the wet for the first time; he proved this by registering no less than 56 laps despite the greasy track; frustratingly for the team, new FTR components are arriving late due to the snow hassle which continues to affect the United Kingdom.
At the sharp end, Lorenzo's leading effort was 1'47.423 to see off closest challenger Pedrosa by one second, as the varying weather and resulting modified test plans ensured that lap times were more spaced out than usual.
Márquez, who had been quickest on all three days at Circuit of the Americas, was on this occasion third as he tackled Jerez with MotoGP machinery for the first time. Energy T.I. Pramac Racing Team's Andrea Iannone and the second Yamaha of Valentino Rossi were fourth and sixth, with 2006 world champion Nicky Hayden leading the way for Ducati Team until test rider Michele Pirro progressed up the order to a top five place while running new parts. This left the American in seventh position.
Cal Crutchlow jumped ahead of LCR Honda MotoGP's Stefan Bradl with less than an hour to go, although the German had crashed a short time before; thankfully he was able to walk away from the scene unaided. The third Ducati Team bike of Andrea Dovizioso completed the top ten from Ben Spies' Ignite Pramac Racing Team machine and PBM's ART, ridden by Colombian Yonny Hernández who was the leading CRT representative.
Elsewhere in the CRT ranks, Power Electronics Aspar did not run for the entire day, not least as Aleix Espargaró was set to run a brand-new engine which neither he nor the team intended to risk damaging by going out in the wet. His continued research into a new chassis and swingarm, first used at Sepang, is now put on hold until Sunday.
That left Karel Abraham's Cardion AB Motoracing and the NGM Mobile Forward Racing of Claudio Corti 14th and 15th, the latter experimenting with chassis changes first suggested by teammate Colin Edwards during their previous outing in Malaysia. Between CRT riders and very much out of position was the prototype GO&FUN Honda Gresini of Álvaro Bautista.
Familiar faces Danilo Petrucci and Héctor Barberá were 17th and 18th for Came IodaRacing Project and Avintia Blusens respectively, but the weather encouraged a tough day for newcomers such as Bradley Smith and Crutchlow's teammate in the all-British Monster Yamaha Tech 3 line-up.
The 22-year-old from Oxford completed only 14 laps, which was the second least after compatriot Michael Laverty; still in the early days of development for the in-house PBM chassis, he circulated just 12 times and completed the 23-bike timesheet behind Gresini's Bryan Staring - much happier with handling - and Czech Lukáš Pešek.
Just two days of pre-season action continue ahead of the curtain-raising Grand Prix, under lights in Qatar on 7th April. To the annoyance of the teams in Jerez, more rain is likely to hit the test across Sunday and Monday.