Formula 1's post-season test is underway in Abu Dhabi, with a number of drivers making their debuts for their 2025 teams just two days after the end of the 2024 season.
The rules of the test allow for teams to run two cars, one for a young driver with less than two races experience and one for 2025 tyre testing.
As a result, a number of teams have used the test as a Day Zero for the 2025 season, using the opportunity to get their new drivers in the car ahead of F1's winter break.
After completing a filming day on Monday, former Ferrari driver Carlos Sainz is at the wheel of a Williams again for his first full day with his new team.
Sainz, who will be replaced by Lewis Hamilton at Ferrari next year, wore an all-white race suit and helmet to avoid sponsor clashes arising from the overlap between the remaining days of his Ferrari contract and his Williams debut. Williams junior driver Luke Browning was in the second car.
Due to commercial commitments with Mercedes, no such deal was reached for Hamilton to drive for Ferrari in the post-season test. Instead, the Italian team fielded Charles Leclerc, who will remain at Ferrari next year, and a second car split between Leclerc's brother, Arthur, and Ferrari Le Mans winner Antonio Fuoco.
After four seasons with Red Bull's junior team RB, Yuki Tsuonda finally made his debut with the company's senior team, driving this year's championship-winning RB20.
The test carries extra significance for Tsunoda with sources telling ESPN that Red Bull is set to move on from Sergio Perez over the winter, vacating a space alongside world champion Max Verstappen in 2025.
Liam Lawson is at the top of Red Bull's list to replace Perez, but he was driving for RB in the test alongside Honda Super Formula driver Ayumu Iwasa, who has strong ties with Red Bull's engine supplier.
As recently as Sunday, Red Bull team principal Horner said both Lawson and Tsunoda would be candidates for promotion in 2025 "in the event that anything were decided with Checo [Perez]."
The second Red Bull test car is being driven by Formula 2 runner-up Isack Hadjar, who is expected to be drafted in at RB in 2025 to replace Lawson if the New Zealander lands the drive alongside Verstappen.
After striking a deal with his former team Alpine, Esteban Ocon was testing for new team Haas as a tradeoff for vacating his Alpine car at the final race for Jack Doohan.
Ocon was driving the 2025 tyre testing car, but rather than being joined by his 2025 teammate Oliver Bearman, who was ruled out of the young driver test after competing at three races this year as a substitute driver, he was joined by Ryo Hirakawa, who has links to Haas through its new technical partner Toyota.
Sauber was fielding its all-new driver line-up for 2025 in Tuesday's test, with Nico Hulkenberg making the switch from Haas and Brazilian Gabriel Bortoleto making his debut for his new team after winning the F2 title last weekend.
Following his debut F1 race in Abu Dhabi, Doohan was driving for Alpine alongside newly-signed reserve driver Paul Aron.
McLaren, which will keep the same driver line-up next season, opted to split its 2025 tyre testing car between Lando Norris and Oscar Piastri, while McLaren IndyCar driver Pato O'Ward is in the second car.
Mercedes had planned to field Hamilton's replacement Andrea Kimi Antonelli in the test, but the 18-year-old is still recovering from an illness that prevented him from competing in his final F2 race at the weekend.
Mercedes junior and simulator driver Fred Vesti took Anotonelli's place alongside George Russell in the 2025 tyre test car.
Aston Martin opted to allocate its cars to two of its junior drivers, with reserve Felipe Drugovich in one and upcoming American F2 driver Jak Crawford in the other.
The test will run until 6 p.m. local time and represent the last on track running of the 2024 season.