Max Verstappen was fastest for Red Bull ahead of Renault's Daniel Ricciardo as Mercedes struggled in a surprising conclusion to Friday's second free practice for the Belgian Grand Prix.
The Dutchman clocked a best lap in one minute and 43.744 seconds to outpace the Australian by 0.048 seconds, leaving championship leader Lewis Hamilton down in third for Mercedes, 0.096 adrift of the pace-setter.
Hamilton, the six-time champion, was on hard tyres and struggled to find the dominant form of recent meetings. He wound up third, ahead of Alex Albon, in the second Red Bull, Sergio Perez of Racing Point and Mercedes team-mate Valtteri Bottas.
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.
The rest of the top 10 were all within eight-tenths of a second of Verstappen, a tight bunching of times that suggests a close contest in Saturday's qualifying and Sunday's race if the cooler conditions continue.
It was a disappointing day for last year's winner Charles Leclerc and Ferrari. Leclerc finished up 15th with four-time champion Sebastian Vettel 17th.
On a cool afternoon with rain threatening in the Belgian Ardennes, the air temperature was 16 degrees Celsius and the track only 23 degrees when the session began with most teams slow to send their cars out first.
Eleven minutes passed before Antonio Giovinazzi ventured out in his Alfa Romeo. If there were had been spectators, they would have had every reason to complain at the lack of action.
Mercedes team chief Toto Wolff suggested after the morning's opening session that after the heat wave at Silverstone and Barcelona and on the longest circuit on the calendar, his team was having trouble finding a balanced set-up.
Verstappen, taking advantage of Red Bull's high down-force set up, topped the times and stayed there ahead of Ricciardo's Renault with half an hour remaining. Soon after this, the Australian was requested by Renault to pull up and park on the Kemmel straight and a Virtual Safety Car (VSC) was deployed. The team said his car had lost hydraulic pressure.
As soon as this was resolved, an advertising hoarding flew on to the track close to Eau Rouge, causing a red flag stoppage for a further two minutes ahead of a final flurry of action.