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Sainz will join the Grove squad on a multi-year contract for 2025 after losing his Ferrari seat to Lewis Hamilton.
The Spaniard’s future was announced on Monday by the team and will see him partner Alex Albon in place of Logan Sargeant.
Sainz had been a key player in the driver market since effectively becoming a free agent in February.
The three-time race winner was the most highly sought-after driver available, with Audi (Sauber), understood to have made an offer.
Discussions with Williams began at the end of last year, before news of Hamilton’s Mercedes departure (and therefore Sainz’s Ferrari release) were known.
“The conversation has been many months,” Vowles told selected media, including Speedcafe.
“It’s been a bit more public than I would normally do with a driver discussion, but actually started way back in Abu Dhabi last year.
“The message I gave to him and to his family at the time is no different to the message I gave him last weekend in Spa.
“I believe that’s what’s won it; from the beginning, I gave him warts and all what’s going to happen.
“We are going to go backwards, here’s why. Here’s what we’re investing in, here’s what’s coming, here’s why I’m excited about this project.”
According to Vowles, Sainz was almost across the line ahead of the Spanish Grand Prix, but a dismal weekend for the team there created some hesitation.
Conversations continued, with he Williams boss eventually securing a driver he considers among the best in F1.
“I rate him as one of the top four drivers, if not at times the number two driver on the grid,” Vowles opined.
“Why wouldn’t you want that in your stable?
“My view of things is that fundamentally competitors are getting closer and closer.
“The marginal difference that a driver can make, and I don’t just mean in performance terms.
“Look at Carlos, and look at every team he has been in, they have improved significantly, and I get why after spending the last nine months talking to him, at least weekly, if not daily.
“He is a performance machine. He absolutely will do everything it takes within his power to not transform just himself but the team around him as well at the same time.
“That’s powerful. That’s worth more than what more than what we can drive the car… he moves the team forward by the same amount.”
Sainz’s arrival at Williams creates an enticing combination with Albon, himself a highly rated driver only starting to come into the prime of his career.
The Spaniard’s arrival is also on-brand for Vowles, who has made no secret of working to secure the future competitiveness of the operation since arriving as team principal at the start of last season.
New regulations for 2026, ongoing investment behind the scenes at the team, and a far stronger driver pairing – arguably the strongest it has had since it paired Felipe Massa with Valtteri Bottas (2014-2016) – leaves the squad in a promising position for the medium-term.
