• Max Verstappen was the fastest driver in the second practice session at Spa-Francorchamps.
• It was the first time the Belgian driver was fastest in second practice since the Styrian Grand Prix.
• The Ferraris were disappointing in both practice sessions.
Max Verstappen topped the times for Red Bull on Friday, but then forecast that he would not be fast enough to stop Mercedes from overhauling him to take pole position for the Belgian Grand Prix.
The Dutchman was quickest ahead of Renault's Daniel Ricciardo as Mercedes struggled in a surprising conclusion to Friday's second free practice.
However, he made clear he expected the champions to respond and take over at the top on Saturday.
"I think they still have some problems with the balance so I expect them to be strong tomorrow," said Verstappen.
"You can also see that they are very fast in the long runs. I don't expect to fight for pole position, but I hope to be close to them."
The Dutchman clocked a best lap in one minute and 43.744 seconds to outpace Ricciardo by 0.048 seconds, leaving championship leader Lewis Hamilton down in third for Mercedes, just 0.096 adrift of the pace-setter.
The six-time world champion, on hard tyres, had struggled to find the dominant form of previous recent race meetings.
But he wound up ahead of Alex Albon in the second Red Bull, Sergio Perez of Racing Point and his own Mercedes team-mate Valtteri Bottas who was celebrating his 31st birthday.
"It's still early to say anything about it, but from our side this was a good start," added Verstappen.
"There are still some things we need to work on for tomorrow, but overall I am very satisfied."
Lando Norris was seventh in the leading McLaren ahead of Esteban Ocon in the second Renault, Carlos Sainz in the second McLaren and Pierre Gasly of Alpha Tauri.
Gasly had earlier walked on the circuit to leave flowers at the scene of the crash that claimed the life of his friend Anthoine Hubert during an F2 race last year.
The top ten were all within eight-tenths of a second of Verstappen, a tight bunching of lap times that suggests a close contest is in prospect for Saturday's qualifying and Sunday's race if the cooler conditions continue.
"It's very close out there," said Hamilton.
"I think the Red Bulls are just a little bit quicker at the moment, but even Racing Point I think is right with us and then also Daniel Ricciardo and Renault are very close.
"That makes it exciting. We've got some work to do, that's for sure, to dial in the car a bit more, but it doesn't feel bad at all.
"As I anticipated coming, it's only the seventh race, the field is going to get closer as we continue to develop, the whole of F1 develops and it's already showing that this weekend.
"We seem to be so close with everyone and we've got to understand why -- whether they've taken a step up or we've taken a step back or if it's just track layout or down-force levels.
"Who knows? We'll work on that, but I'm excited for an interesting race."
It was a disappointing day for last year's winner Charles Leclerc and Ferrari as he and four-time champion Sebastian Vettel failed to make any impact on the top ten and finished up 15th and 17th, separated by the Williams of George Russell.
Mercedes team chief Toto Wolff had suggested earlier following the morning's opening free practice that his cars had found the much colder conditions than the heat-waves experienced at Silverstone and Barcelona challenging as they worked to find a set-up for the calendar's longest lap.
"It's so rapid here," said Hamilton. "Every year our cars get faster and faster, particularly with this generation of car.
"I mean, already last year, the car was pretty spectacular here but, it's phenomenal the speeds we can go through corners."