Spa veteran Kimi Raikkonen kept Ferrari on top of the Belgian Grand Prix practice timesheets on Friday, just ahead of Formula One world championship leader Lewis Hamilton's Mercedes.

Finnish driver Raikkonen, four times a winner at one of the most exhilarating and daunting tracks on the calendar and chasing the 100th podium finish of his career on Sunday, produced a time of 1 minute 43.355 seconds under leaden skies.

Hamilton was 0.168 seconds off the pace, with Finnish team mate Valtteri Bottas - who will start the race from the back of the grid because of engine penalties - third fastest.

Raikkonen's team mate Sebastian Vettel had been top in the morning session - the first since the three-week August break - in 1:44.358. The German, 24 points behind fellow four times champion Hamilton with nine of 21 races remaining, was fifth in the afternoon.

Red Bull's Max Verstappen, with tens of thousands of his Dutch fans expected to make the short trip across the border, was second and fourth respectively in the two sessions.

Hamilton, last year's winner in the Ardennes forests from pole position, had been third in the morning.

Vettel expressed optimism on Thursday that Hamilton could be reeled in over the remaining races, some of which should favour Ferrari more than Mercedes.

"Points-wise we know that we are a little bit behind but I dont think its anything that we cant do," the German told reporters.

Both Vettel and Hamilton will have upgraded engines, with Mercedes saying the Briton had a new turbocharger, engine and MGU-H and MGU-K motor generator units in his car.

Those parts were within his allocation for the season but team mate Bottas incurred penalties after being given a completely new power unit that exceeded his allowance.

Renault's Nico Hulkenberg will also start from the back of the grid for the same reason.

Britain's Lando Norris made his grand prix weekend debut in the morning, taking Fernando Alonso's McLaren for the session, with the 18-year-old team reserve lapping faster than under-pressure Belgian race regular Stoffel Vandoorne.

Norris, who is also chasing the Formula Two title, lapped in 1:47.364, against Vandoorne's 1:47.452.

Vandoorne drove only 13 laps compared with Norris's 26, however, kept in the garage as the team investigated a possible leak on the car.

The Belgian was slowest in the afternoon, with Alonso returning to post the 16th best time.