The Australian started on pole, converting it into his second Feature Race victory of the year, with Theo Pourchaire second and Jehan Daruvala third.
“I'm very happy with our second Feature Race win,” Piastri said.
“It was a tough race, I had Pourchaire behind me for the whole time and really had to push the whole way to keep him there.
“It was fun even with a lot of pressure, but I'm very happy to have converted the pole into another win and added up to the championship lead which is even better.”
The Prema Racing driver crossed the line ninth in the rescheduled Race 1 on Saturday afternoon, just outside the points as he chased Richard Verschoor to the line.
Dan Ticktum claimed honours in that encounter from Juri Vips and Robert Schwartzman.
It was the only sprint race of the weekend, Race 2 cancelled due to heavy rain and the reshuffled programme on Saturday.
“It wasn't a great race, and it was always going to be tricky given the conditions and starting from 10th,” Piastri said of his Saturday performance.
“I think I made a few too many mistakes, which is unfortunate, and I could have got a couple of points, but it is what it is.”