“It’s been a very long two weeks for everyone back at the factory,” Red Bull’s race engineering co-ordinator Andy Damerum told the sport’s official website, F1.com.
But he added: “I have to say that, while it might not appear so on the surface, there has been a big improvement since Jerez.
“We are obviously playing catch-up and, for us, this was very much like a first test.
“We were sidelined by a mechanical gremlin but one unrelated to the issues in Jerez. That’s the way of testing.
“All these problems have to be ironed out and only reveal themselves as you progress. We’re itching to get back on the track again.”
Wednesday’s opening day honours were taken by Hulkenberg, the German returning to Force India after a brief spell with Sauber, posting a quickest time of 1 minute 36.880 seconds after completing 78 laps.
“A very positive and satisfying day,” said the pacesetter.
Ferrari’s Fernando Alonso (1:37.879) came in next, one second behind, with Lewis Hamilton (1:37.908) third in his Mercedes.
Hamilton reported: “We’re not aiming to be top of the timesheets at the moment, it’s more about ticking the reliability boxes, but we can be happy with where we are before the halfway point of the pre-season tests.”
In fourth, carrying on his impressive showing in Jerez, was McLaren’s Danish rookie, Kevin Magnussen, who had the satisfaction of finishing one place ahead of Vettel (1:40.224).