Lewis Hamilton was third fastest in final practice as Ferrari threw down the gauntlet for the Belgian Grand Prix.
Kimi Raikkonen posted the quickest lap of the weekend to finish 0.197 seconds clear of his team-mate Sebastian Vettel.
Vettel, who heads Hamilton by 14 points in the title race, was just one thousandth of a second faster than Hamilton to set up a potentially thrilling qualifying session in the Ardennes.
The fast-sweeping nature of the famous Spa-Francorchamps circuit had been expected to play to Mercedes’ strengths, and so it proved on Friday with a dominant Hamilton topping the time sheets.
But Ferrari’s pace on Saturday morning suggested they could be in a position to take on Hamilton, who will match Michael Schumacher’s all-time pole record if he is quickest in qualifying.
Max Verstappen, cheered on by a large army of Dutch supporters who have made the short journey across the Belgian border, was fourth quickest, albeit more than one second adrift of Raikkonen’s best time.
Valtteri Bottas was fifth for Mercedes, 1.3 seconds slower than Raikkonen, with Red Bull’s Daniel Ricciardo sixth.
British driver Jolyon Palmer is one of only two drivers yet to score a single point this season, but he has enjoyed a strong weekend thus far.
The 26-year-old Englishman was seventh in final practice, half-a-second quicker than his Renault team-mate Nico Hulkenberg.
Fernando Alonso was 10th for McLaren, while his team-mate Stoffel Vandoorne, who will serve a whopping 65-place grid penalty following a series of engine penalties, was 14th.