Mark Smith due to join Sauber on July 13 as team's first technical director since 2012.
German picks up where he left off during Austrian Grand Prix by ending the test over eight tenths clear of Ferrari's Esteban Gutierrez.
Finn clocked a 1:10.029 in the FW37, while Nico Rosberg finished the morning third for Mercedes and Fernando Alonso managed just 17 laps in the McLaren MP4-30.
Mercedes junior keeps the world champions top of the timing screens after day of testing hampered by rain at Red Bull Ring.
Swede started before the lights go out and moves on to five penalty points - more than any other driver.
Italian youngster handed another day behind the wheel after driving for both Sauber and Ferrari at the post-Spanish GP test.
Sauber will not use Ferrari's updated engine at the Canadian Grand Prix and will instead wait until the Belgian Grand Prix to introduce an upgraded power unit.
Italian will take over from Marcus Ericsson in FP1 before driving in the second day of the test which takes place in Barcelona the week after the grand prix.
Monisha Kaltenborn is wary of making too many changes to the regulations in 2017 as the sport targets 1000bhp engines and a new "spectacular formula".
ESPN rounds up all the reaction from up and down the paddock after the Bahrain Grand Prix
ESPN rounds up all the reaction from up and down the paddock after qualifying for the Bahrain Grand Prix.
ESPN rounds up all the reaction from up and down the paddock after qualifying for the Chinese Grand Prix
World champion tops his third practice session of the weekend as Mercedes shows its one-lap advantage over Ferrari in China.
All the reaction from up and down the paddock after Friday practice in Shanghai.
Sauber boss Monisha Kaltenborn thinks F1 has lost its direction and is losing its connection with fans.
ESPN rounds up all the reaction from up and down the paddock after the Malaysian Grand Prix
A look at the main talking points ahead of the Malaysian Grand Prix as F1 looks to bounce back from a damp squib in Melbourne
Ferrari junior to take over from Felipe Nasr in first practice session of Malaysian Grand Prix
Team says it refuses to engage Dutchman in 'a mud fight via the media' and insists it considers the matter closed
Dutchman admits his F1 career is 'probably over' and takes a swipe at Sauber team boss Monisha Kaltenborn in remarkable statement
Australian court grants driver legal right to take part in season opener
Giedo van der Garde will find out on Wednesday whether a Victoria court has ruled in favour of his legal challenge to drive for Sauber at this weekend's Australian Grand Prix
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