Formula 1 has completed day one of its highly secretive first day of preseason testing -- a day so secret that even each team's quickest lap times have been hard to come by.
Media and fans are banned from F1's first week of testing in Barcelona, a test arranged arranged to allow teams to test cars which are radically different under F1's revamped 2026 regulations, which features major changes on both the aerodynamic and engine side of the car, before two official tests in Bahrain in February.
Some time after the final lap of the day, sources confirmed to ESPN Red Bull's Isack Hadjar set the fastest time on Monday, with George Russell 0.527s behind him in a time also set in the final hours of the day.
Many in the paddock expect Mercedes to be the strongest team under the new regulations and those in attendance said the W17 looked impressive immediately.
Russell's teammate Antonelli had been the first car out on track when the session opened on Monday morning. Mercedes had often made a habit of going straight out on track during tests during their dominant spell of championships between 2014 and 2021.
Formula 1's teams went to great lengths to maintain the secrecy of the first test -- security guards strictly enforced the rules outside the circuit, while live timing information which briefly leaked out from the circuit was shut down during the first day.
Teams are in charge of what information they reveal about their own running, with many reluctant to release lap times without the context of their day in full, making sense of an early competitive order difficult.
Headline times are often difficult to make sense of during an open test -- that is even moreso the case in private, when basic context of track conditions and time of lap is harder to come by. However, Red Bull and Mercedes will both have been buoyed to have turned in early quick laps.
ESPN can confirm seven teams ran on the opening preseason day -- 2025 entrants Mercedes, Red Bull, Racing Bulls, Alpine, Haas among them.
New teams Audi -- who have completed their takeover of Sauber and are competing with their own engine this season -- and Cadillac, who become the sport's 11th team this year, were also on track.
Valtteri Bottas took the Cadillac on track for the first time in an all-black testing livery -- the team is set to unveil its 2026 colours in an advert during the Super Bowl.
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"Priority list now is get more laps each day, think I got a bit more than 30 laps today, some of them just installs and stuff, but we need to get some proper running in, get the mileage in, and make sure we get a reliable package for race one." Bottas told F1 TV.
Reigning world champions McLaren and Ferrari have already confirmed they will start their respective programmes on Tuesday.
Aston Martin looks set to fall at least one day short of the allotted three - the team released a statement on Monday saying it intends to be on track on Thursday and Friday, raising questions about its own readiness levels for the new year.
Aston Martin has invested heavily in this current regulation cycle in a bid to become world champions.
Williams has already confirmed it will skip the test altogether after hitting delays with the build of its new car.
"Now that we are running, we are discovering issues, problems we need to solve, every lap, every run. But that's what we are here for, right?" Haas boss Ayao Komatsu said.,
"At least we are running, that means we are gathering data, we are identifying issues that we need to solve. Still, obviously we just got started, but I'm very, very happy how we started."