Max Verstappen picked up where he left off at the Red Bull Ring to again top the timesheet on the team's home circuit on the opening day of the venue's second event, the Austrian GP.
Last Sunday's runaway race winner set a morning pace of 1:05.143 to outpace Ferrari duo Charles Leclerc and Carlos Sainz, who each displayed strong race pace here last Sunday, by two tenths of a second.
Mercedes have used the few days between races to try and find ways to improve the pace of their W12 around a circuit that has not always historically favoured them - and certainly did not last week - with Lewis Hamilton again returning to the team's factory to help with the efforts.
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Valtteri Bottas was their lead runner in opening practice with the fourth-quickest time, while Hamilton was seventh.
Japan's Yuki Tsunoda, a points finisher last week, was fifth fastest ahead of Alfa Romeo veteran Kimi Raikkonen.
Lance Stroll was one of last week's top performers but the Canadian driver spun his Aston Martin twice during the opening session, once at Turn Six and then at the final corner.
"I had no grip - these tyres are done," reported Stroll over the radio after the latter incident.
The session saw the debut of Pirelli's prototype rear tyres, which feature a more robust construction which F1's tyre supplier says "can guarantee even greater levels of integrity under the extreme conditions that can be generated by the current cars".
The test comes after the tyre blowouts for Verstappen and Stroll at the Azerbaijan GP last month, which were followed by the introduction of more stringent checks and parameters concerning how tyres are run by teams on track.
Each driver must complete 12 laps on the tyre, which Pirelli plan to introduce at the British GP in two weeks' time if Friday's running is successful.
Meanwhile, Friday morning's drivers included a latest run-out Britain's Callum Ilott, a Ferrari protege who finished 16th quickest for customer team Alfa Romeo, and an F1 race weekend debut for China's Guanyu Zhou.
Taking over Fernando Alonso's car for the opening session at Alpine, Zhou ran strongly and finished within 0.4s of Esteban Ocon in the sister car.