Bottas, who won the season-opening Austrian Grand Prix in July, is third 43 points behind his team mate.
With 11 of this season's 17 races still to run, he needs to start turning the tide if he is to have any hope of denying Hamilton a record-equalling seventh title at the end of the year.
Sergio Perez was fourth in his Mercedes-inspired Racing Point ahead of Canadian team mate Lance Stroll.
Alexander Albon, who drove his first race for Red Bull at Spa last year after moving up from junior team Toro Rosso now rebranded Alpha Tauri, was sixth.
Esteban Ocon, who made his Formula One debut in Belgium in 2016, was seventh for Renault ahead of McLaren's Carlos Sainz.
Daniel Ricciardo was ninth in the other Renault with Briton Lando Norris rounding out the top 10 for McLaren.
Ferrari, winners for the last two years in Belgium, continued to struggle. Charles Leclerc, who took his maiden Formula One win at the Spa circuit last year, was only 14th ahead of departing four-time champion Sebastian Vettel in 15th.
The session was largely uneventful.
All cars carried special stickers in memory of Frenchman Anthoine Hubert, who suffered fatal injuries in a Formula Two race on the Saturday of the Belgian Grand Prix weekend last year.
Birthday boy Bottas fastest in opening Belgian GP practice
Image: Reuters
Valtteri Bottas set the pace ahead of Mercedes team mate Lewis Hamilton in Friday's opening practice session for the Belgian Formula One Grand Prix.
The Finn, who turned 31 on Friday, lapped the seven-kilometre Spa-Francorchamps track in one minute 44.493 seconds, beating Hamilton by just 0.069 seconds on a cool and cloudy morning.
Max Verstappen was best of the rest again in third for Red Bull.
The Dutchman, who has a Belgian mother and is missing the support of his army of orange-clad supporters who usually throng the grandstands at Spa, was hot on the Mercedes pair's heels, ending the session just 0.081 seconds off the pace.
Hamilton, winner of four of this season's six races, leads the overall standings by 37 points from Verstappen.
Bottas is feeling the heat in Mercedes' black overalls
Bottas, who won the season-opening Austrian Grand Prix in July, is third 43 points behind his team mate.
With 11 of this season's 17 races still to run, he needs to start turning the tide if he is to have any hope of denying Hamilton a record-equalling seventh title at the end of the year.
Sergio Perez was fourth in his Mercedes-inspired Racing Point ahead of Canadian team mate Lance Stroll.
Alexander Albon, who drove his first race for Red Bull at Spa last year after moving up from junior team Toro Rosso now rebranded Alpha Tauri, was sixth.
Esteban Ocon, who made his Formula One debut in Belgium in 2016, was seventh for Renault ahead of McLaren's Carlos Sainz.
Daniel Ricciardo was ninth in the other Renault with Briton Lando Norris rounding out the top 10 for McLaren.
Ferrari, winners for the last two years in Belgium, continued to struggle. Charles Leclerc, who took his maiden Formula One win at the Spa circuit last year, was only 14th ahead of departing four-time champion Sebastian Vettel in 15th.
The session was largely uneventful.
All cars carried special stickers in memory of Frenchman Anthoine Hubert, who suffered fatal injuries in a Formula Two race on the Saturday of the Belgian Grand Prix weekend last year.
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