Issues with the balance of the W08 EQ Power+ has been the thorn in the sides of Lewis Hamilton and Valtteri Bottas, with the car's weight being at its root.
Hamilton, speaking before the Spanish Grand Prix this weekend, said the form Bottas had shown came as no surprise while insisting their Mercedes team had no intention of favouring either driver in their battle for the title with Ferrari's Sebastian Vettel.
Felipe Massa was the early pacesetter as the cars made their way around the Circuit de Barcelona-Catalunya, though he was soon surpassed by Valtteri Bottas and Kimi Raikkonen before Hamilton left both in his wake.
Bottas was expected to be no more than an able deputy to Hamilton at Mercedes after he was chosen to replace Nico Rosberg - but he has outqualified his team-mate at two grands prix in succession and finished 36 seconds clear of the British driver at the last round.
"Getting that first win definitely gives me a lot of confidence", he said after Sochi.
"Of course he's a credible competitor [for the title], he was when he joined", Hamilton said.
"For many reasons in Sochi it was hard to overtake; tyres lasted well so you could push whether you were in front or behind, track layout doesn't make it easy to follow close which makes it quite hard to get close for the straight line".
"Fair play to him". My main focus is on this year.
Hamilton was a distant fourth in Russian Federation and after the race appeared at a loss to explain why he was so slow at a circuit at which he often excels.
Mercedes, like all teams, arrive at Barcelona with auto upgrades and Bottas reckons the battle with Ferrari will continue to be defined by fine margins.
"I'm not happy", Vettel said.
Though from past seasons, Force India, who are now occupying fourth place in the constructors championship, have been the team to succeed with radical upgrades while the others tend to fade away. "So, no different for me, I've just got to do a better job".
"We've since been unpicking what happened to understand why we couldn't get the auto in the right window so he could feel comfortable with it".
In Russia, Hamilton was out of contention well before Bottas fended off Vettel for the victory.
In 78 races for Williams before joining the constructors champions this year the Finn had never won a race, twice finishing runner up. Raikkonen won the season-opener with Renault and Vettel won twice with Red Bull. Vettel went on to dominate, winning the last nine races to easily clinch the world title for the fourth time. It is quite nice but after a few days, it is done. There had been no discussion at Mercedes of one driver ceding to another. "I struggled a bit with the auto balance in general so conditions today seemed to be quite challenging".