Froome and Porte were 19 seconds clear of Adam Yates (Orica-GreenEdge) and Dan Martin (Etixx-QuickStep) with Contador (Tinkoff) a further two seconds back.
With two stages left of the traditional Tour de France tune-up race Froome holds a seven-second lead over Porte.
Saturday's stage is likely to decide the race with several tough climbs sandwiched into the 141km ride from La Rochette to the ski resort of Meribel.
(Reporting by Martyn Herman, editing by Ed Osmond)
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