Valtteri Bottas edged out Max Verstappen to finish fastest in opening practice for the Portuguese Grand Prix.
Bottas lapped the Portimao circuit just 0.025 seconds quicker than Red Bull's Verstappen as Lewis Hamilton finished fifth.
The seven-time world champion described his Mercedes as "undriveable", and ended the first session three tenths adrift of team-mate Bottas.
Formula One is in the Algarve for the third round of the season, with Hamilton holding a slender one-point advantage over Verstappen.
At the last race in Imola, the British driver finished behind Verstappen after slipping off the road in damp conditions.
On Friday, at a track nicknamed the rollercoaster due to its undulating, fast-sweeping corners, Hamilton was disappointed with the handling of his Mercedes.
"It is pretty undriveable," said the world champion following his early running. "We need to box and make some serious changes."
Hamilton later returned to the circuit but he was unable to put together a clean lap, finishing adrift of Bottas, Verstappen, Sergio Perez in the other Red Bull, and Ferrari's Charles Leclerc.
Verstappen might have finished within a quarter of a tenth of Bottas but the the Dutchman also struggled with his tyres.
"What a joke," said Verstappen over the radio. "I cannot see where I am going because of the vibrations."
Told by his team he could stop if it was a safety concern, he replied: "It is OK, I will just visit the dentist after the weekend is over." Verstappen later complained that his car was "impossible to drive".
Following his 200mph crash with George Russell in Imola, Bottas will take some comfort from topping the time charts in the first action of the weekend.
Russell, who issued an apology to Bottas for his conduct after the high-speed coming together, finished an impressive seventh in his Williams, only eighth tenths off the Finn.
Ilott, 22, was announced as a reserve driver for Alfa Romeo earlier this week and is expected to take part in a number of opening practice sessions throughout the season.