The Dutchman made the most of an upgrade package on his RB18 to set a time that was 0.695 seconds faster than Ferrari's Charles Leclerc, as the pair rolled out a series of fast laps late on.
But while Verstappen's effort offered some first clues about the true potential of the Red Bull, the world champion reckoned that neither he nor anyone else was pushing flat out just yet.
"Nobody is in full beans qualifying spec at the moment," he told F1.TV as the three-day Bahrain test came to an end.
Verstappen explained that the best aspect of the day was not being fastest but actually that he was able to start understanding the different tyre compounds, as well as confirm that the Red Bull sidepod developments had worked.
"I think the main focus was about the tyres that we're going to use next week," he said. "The car was feeling alright, and we went through our programme we planned to do, and that's always positive, I think."
He added: "Of course, the car is better in low fuel, and naturally on high fuel the car is a bit lazy and it doesn't really do a lot. But it's the same for everyone. I think the new parts which were on today, they also worked well, which you always hope for."
Asked if the updates would now be on the car for next weekend's opening grand prix of the season, Verstappen said: "They work, so then hopefully, we'll keep them on."
Verstappen's form in Bahrain has confirmed Red Bull's position as one of the pre-season favourites, with Ferrari also looking incredibly strong.
And while other teams like Mercedes and McLaren have had more difficult times on the hotter and bumpier Bahrain circuit, Verstappen says that Red Bull appears to have made a step forward compared to the different conditions in Barcelona.
"Well compared to Barcelona more of it is just the track, you know, the rough surface, warmer conditions and that," he said. "The layout of the track makes it a completely different feeling compared to Barcelona.
"But yeah, of course, I think we learned a lot more about the car, so we made the car faster. And I think that's what you want."
