Red Bull's Max Verstappen, who suffered a blow to his already slim championship hopes when he was taken out of last weekend's Austrian Grand Prix by Mercedes' Kimi Antonelli, was only 10th fastest, complaining about the balance of his car.
Red Bull are one of a number of teams with revised floors for this event, the others McLaren, Aston Martin, Haas, Williams and Sauber.
At Aston Martin, Fernando Alonso used the new floor in the first session, to end up 11th fastest, while team-mate Lance Stroll ran the previous specification for comparison and ended up just one place behind.
British-Swedish driver Arvid Lindblad, 17, replaced Yuki Tsunoda for first practice and was 14th.
In warm temperatures and in front of a large crowd, Alpine's Pierre Gasly and Sauber's Gabriel Bortoleto both had huge spins at the ultra-fast Copse corner, remarkably without going off track and damaging their cars.