A win on Sunday will take him into the championship lead.
And it will be significant for Hamilton on another level, as he has never won the Spanish Grand Prix. Like Sebastian Vettel who has won here only once and never been on pole, this is something of a bogey circuit; things have never gone quite right here.
The challenge to Hamilton from Rosberg will be more akin to what happened in Bahrain – the best race of the season so far – than China, where Hamilton dominated. This is a strong track for Rosberg, who was on pole here last season.
It will also be an interesting indicator of Mercedes’ progress as the car chewed up its tyres terribly last year, promoting Hamilton to describe it as the one of the worst races of his career.
Vettel has a different chassis this weekend, the Red Bull team is giving him all the help it can as he seeks to regain competitiveness after a poor start to the season. Daniel Ricciardo has been faster than his champion team mate on several occasions, even causing the team to ask Vettel to let him through in the last two races.
That is a pattern Vettel is keen to arrest as soon as possible.