Lewis Hamilton lapped six tenths of a second faster than Mercedes team mate Nico Rosberg in a twice halted Belgian GP practice session.
Marussia reinstated British driver Max Chilton for the Belgian GP, a day after the team dropped him for American Alexander Rossi.
Tony Stewart will skip this weekend's Nascar race, the third event he has missed since being involved in a fatal on-track accident.
Formula One team say 16-year-old has already shown he's championship material and not a "risk" as rivals say.
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