Robert Kubica finished ahead of Williams team-mate Lance Stroll on his return to Formula One at the Spanish Grand Prix, but it was still a disappointing practice session for the Pole.
The 33-year-old missed out on a race seat this season, but was named as reserve and development driver for the British team.
Barcelona provided his first opportunity to appear at a grand prix weekend since Abu Dhabi in 2010, Kubica stepping in for Sergey Sirotkin.
However, it was a tough 90 minutes for Williams as they posted the slowest two times, Kubica finishing a second clear of Stroll.
While Williams were frustrated, there was plenty of encouragement for Mercedes as Valtteri Bottas found impressive pace to top the timings by eight tenths from Lewis Hamilton.
There was also promise for a new-look McLaren with Fernando Alonso sticking close to Ferrari and Red Bull, who saw Daniel Ricciardo crash his newly upgraded RB14.
"Thankfully the damage is pretty light," Red Bull team principal Christian Horner said of Ricciardo's car. "Thankfully it was the old front wing so he's a lucky boy.
"He will be in the right specification for the next session."
