Pirelli has elected to bring tyres with a thinner tread to Barcelona - as well as the Paul Ricard and Silverstone F1 races later in the year - and Hamilton said he, like Vettel, noticed the difference with the Spanish GP specification.
After a poor early attempt by both Ferrari drivers, Vettel and Kimi Raikkonen recovered in the closing seconds to finish third and fourth.
Out came the safety auto with Hamilton then executing a masterful restart to leave Vettel for dust before establishing a comfortable cushion over his rival.
Vettel had beaten Valtteri Bottas into the first corner and stayed ahead after the first round of pit-stops, but Ferrari's decision to stop for a second set of medium tyres in the final stint ultimately relegated him to fourth. Raikkonen had yet to pit on lap 24 when Verstappen made the observation, and less than a lap later the Finn slowed and had to return to the garage.
The early stop nearly certainly informed Ferrari's decision to pit Vettel under the VSC.
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"Bottom line is we're not quick enough to win. We just need to understand why". Verstappen has good memories here, as he captured his first ever F1 win in Barcelona in 2016, while Ricciardo finished third at this race a year ago.
He couldn't. The Dutchman kept his cool for a first podium finish of the season - a much-needed result following an incident-packed start to the 2018 campaign, which included his clash with Ricciardo last time out in Baku.
Kevin Magnussen converted the encouraging pace Haas has displayed all weekend to qualify best of the rest in seventh, edging out local hero Fernando Alonso, who put McLaren through to Q3 for the first time of the season. Hartley will start the race from the back of the grid. Every time that I will have a auto that is close to victory, I will go for it.
All were unhurt, although Grosjean had to go to the medical centre for a precautionary check because the incident triggered the high impact alert in the auto.
Grosjean's spin and crash were being investigated by the race stewards after the race. Hulkenberg was withering in his assessment of the Frenchman's driving. He didn't look great in that scenario. "He has to look at it and do some work on himself".
Monaco in two weeks' time. "Could be P1 or P6 (qualifying), but that makes it exciting". "I felt really comfortable in the vehicle". This is when we're going to start to continue to apply the pressure. A one-two for the team.
The Kiwi had a very heavy shunt in the final practice session and the mechanics were still working on his vehicle when qualifying began.