A late downpour meant the field launched for the Saturday race behind the safety car ahead of a rolling start.
Drivers reported improved conditions to immediately ditch full wet tyres for intermediates, but 10 drivers stayed out to avoid losing time in a busy pitlane.
Piastri pitted straight away for intermediates, while Red Bull took the "safer call" by delaying Verstappen's stop for another lap - even if he was eventually held in the pit box for a longer 3.2 seconds to avoid clipping Lando Norris.
This handed Piastri a lead of 1.7s ahead of the safety car returning when Fernando Alonso crashed out. But Piastri only survived in the lead until lap six, with Verstappen catching a tow through Eau Rouge to pass down the Kemmel Straight on his way to securing a comfortable victory.
Piastri reckoned he was "no match" for the chasing RB19. The Australian said: "We tried our best. We boxed when the safety car came in. We led a few laps. But we were no match for Max."
Piastri reckoned the Alonso-induced safety car might have worked to his advantage as it reduced the amount of time he would have to defend from reigning double champion Verstappen.
However, he then summitted Eau Rouge and realised Verstappen was "basically on top of me already".
Piastri continued: "I thought the safety car was going to be in my favour, as it meant less [sic] laps to try and hold him behind.
Source: Autosport