They rediscovered it around the long sweeps of the 4.6-kilometer long track and unleashed it to devastating effect.
Hamilton was expected to put pen to paper on a new deal at the start of the season, but as of the fifth round of the championship nothing has been announced.
His situation is also a lot more comfortable than Vettel's, with the German - also a four-times world champion - now off the podium for three races in a row.
By contrast, the track at Barcelona is much better suited to the W09 - as today's dominant performance from Lewis Hamilton demonstrated.
"Today, the vehicle and myself, I felt that synergy today, which I hadn't been feeling for the whole year", he said immediately after pulling up in parc ferme. "This is when we are going to start trying to continue to apply the pressure". When the race resumed, Hamilton seemingly got into a groove setting the fastest lap on the seventh lap and began widening the gap between him and Vettel.
Vettel finished fourth at the Circuit de Barcelona-Catalunya, extending his winless run to three races and seeing Lewis Hamilton increase his lead to 17 points in the drivers' championship.
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"Why our vehicle doesn't like to be quick around the corners in Monaco, we haven't found out yet".
The nature of the circuit, the temperatures, the tyres featuring a thinner tread depth than usual, all conspired to drop the W09 into the sweet spot.
Mercedes boss Toto Wolff said: "My mind is already on Monaco and I remember it wasn't a pleasant Sunday night previous year". This shows that Pirelli still needs to make progress in terms of choosing the tyres for the weekends, and I would like to see them have a two-step difference in compounds (so, for example: ultras, soft, mediums).
Hamilton had also won in Azerbaijan two weeks ago, but that victory came mostly because of incidents involving his closest competitors, including a blown tyre for Bottas, a mistake by Vettel and a crash between Red Bull teammates Daniel Ricciardo and Verstappen.
Echoing similar views, Mercedes boss Toto Wolff insisted that neither his team wants to lose Hamiliton, nor he wants to leave the team. Behind them, carnage ensued at Turn 3, with Grosjean, Gasly and Hulkenberg all being involved in a collision that brought out the safety auto. "This is what we will do and we will let you know as soon as possible".