The optimism, however, was short-lived, as the Renault-sponsored gremlins returned with a vengeance on the penultimate day of pre-season running in Bahrain.
Sebastian Vettel hardly turned a lap all Saturday.
But McLaren's Jenson Button says he has sensed some pace from the reigning world champions.
"When they do get reliability, if they do, they'll be very competitive," he said.
Ten days into the winter, a clear pecking order is emerging, and Mercedes is right at the very top.
Lewis Hamilton, however, is rejecting the favourite tag.
"We have a strong package but I don't feel we are guaranteed to be the guys ahead," he said.
But rival big-hitters Ferrari and McLaren are clearly not quite on the Brackley team's pace.
"We need more downforce," Button conceded. And Ferrari's Fernando Alonso admitted the new F14-T "could be better in both performance and reliability".
The situation is giving the other two Mercedes-powered teams, usual midfielders Williams and Force India, an opportunity to scoop up big points at the start of the season.
"At this test the Mercedes is still as quick as it was, and the Williams too, but you also now have the Force India that is competitive," said Button.
Ousted McLaren driver Sergio Perez, now at Force India and quickest on both Thursday and Friday in Bahrain, said: "Right now everything seems possible."