Despite continuing to circulate, Ocon dropped down the order very quickly in the Alpine which does not help the team's hopes of good points having started seventh and eighth.
Ocon's front left tyre slammed into the side of Piastri's McLaren, which somehow escaped without significant damage, while the Frenchman had to retire.
Pierre Gasly kept the team's ambitions alive in ninth having lost out to Sergio Perez's Red Bull.
Yuki Tsunoda completed the top 10 at the end of lap one having worked his way through the field on the opening tour.