Russell made a good getaway from eighth and coming down the hill from La Source he tried to pass the ailing Piastri, the Australian having damaged his car when squeezed between the Ferrari of Carlos Sainz and the inside barrier.
Russell headed to Piastri's left, following Lando Norris, but finding the route blocked, he was forced to back off.
Fernando Alonso, Yuki Tsunoda and Lance Stroll all went past into Eau Rouge, while Alex Albon got a good tow and dragged by at the end of the straight.
Having tumbled to 11th, Russell went for a long opening stint and a single stop, and eventually recovered to sixth.
“It was just very unfortunate in the beginning,” he said when asked by Autosport about his afternoon.
“When Oscar had his failure, I went to the left, sort of misjudged that he can turn, I got boxed in, and lost four or five positions at the start.
“So that kind of dictated my race. If it wasn't for that we would have been P5. I would have liked to think I would have been there with Lewis [Hamilton] and Charles [Leclerc], fighting for the podium.”
Source: Autosport