Antonelli, 25 points clear of closest rival Russell in the Formula One standings, took his sixth pole in 10 rounds at the Belgian Grand Prix, and the 19-year-old Italian will be chasing his sixth win in Sunday's race.
Russell will start third thanks to McLaren's Lando Norris having a 10-place grid penalty after qualifying ahead of him.
“It could be worse, but it could be much better," said Russell. "Yesterday, I was losing eight-tenths on the straights, and today I am losing four tenths. It is a step in the right direction.
"We saw this at Silverstone, and we thought we had discovered the problem. We thought it was the brakes. It wasn’t. Then we thought it was my driving style with the throttle, and I convinced myself that it was something in me, but now we are very confident it isn’t my driving style.
"There is a serious issue at play here, and although the team are working so hard to resolve it, every lap I do and I see I am anywhere between two and six tenths down on the straights, it is pretty infuriating.”
Russell has won twice this season but has also struggled to match Antonelli's pace despite starting the campaign as title favourite.
He said his entire focus since the start of practice was straight-line speed, rather than tyres or general setup.
"We are all trying to solve what is going on...I am watching on my steering wheel that I am losing speed when you are full gas on the straight, and I feel powerless. We don’t know what is going on. I don’t think it is the power unit. There is something slowing me down on the straights.
"The truth is, the battle against my teammate, who is such an incredible driver, who is doing such a great job at the moment, in the best of times, is a tall order. But I feel confident head-to-head I can achieve it," added Russell.
"But when it feels like you are battling with one hand behind your back, it's a challenge".