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.”
He added: “I think one stop around here was a pretty competitive stint. The last stint I felt really strong on the soft tyre. It's just been a really challenging weekend. Glad it's over. A bit of a reset, and go again.”
Russell admitted that his long first stint wasn’t easy, especially when it began to rain and he was on older tyres.
“I mean, it was definitely tricky when the rain started coming down,” he said. “Again, if we knew how much rain there was going to be, we would have pitted onto the soft, and that's when you would have had the biggest difference in those challenging conditions.
“But yeah, all in all, it was a good race. Both the races were competitive, opening that gap to P3 in the championship. But we're still focused on Red Bull, and trying to close that gap."
Russell admitted that he thought McLaren would be faster at Spa, and conversely Ferrari less so.
“They [McLaren] weren't as competitive as I thought they were going be this race,” he said. “I think Charles was maybe a surprise for us. I don't think we anticipated them being so quick.
“But as I said, we're in a good place in the teams’ championship. That looks pretty secure for now. But we're just focused on trying to win a race this year.”
