The primary talking point of the session came just 14 minutes in when Hartley went off heavily into the barriers at Brooklands.
"He's a bit shocked, but OK", said Hartley's team mate Pierre Gasly, whose auto was thoroughly checked afterwards for signs of any similar suspension problem.
Hartley won't be alone in the pitlane when the light go off as he'll be lining up right behind Williams drivers Sergey Sirotkin and Lance Stroll whose cars were also revised after qualifying.
Gasly's early time, prior to Hartley's failure, left him 19th on the leaderboard.
Bottas finished up third fastest in the second Mercedes, more than half a second down on Raikkonen, while FP2 pacesetter Vettel was only fourth. Leclerc managed to remain seventh quickest, behind Max Verstappen and Daniel Ricciardo, but Ericsson dropped back to 10th behind Kevin Magnussen and Romain Grosjean.