Mercedes Formula One driver Nico Rosberg(C) of Germany celebrates with Red Bull Formula One driver Sebastian Vettel of Germany (L) and Williams Formula One driver Valtteri Bottas of Finland after the qualifying session of the Hungarian Grand Prix at the Hungaroring circuit in Mogyorod, near Budapest July 26, 2014. REUTERS/Laszlo Balogh
(Reuters) - Sebastian Vettel is "worn out" after winning four consecutive world drivers' titles to enable Red Bull to reign supreme in Formula One, according to his team chief Christian Horner.
Briton Horner said that exhaustion was a part of the explanation for the 27-year-old German's loss of form in a season dominated by Mercedes and in which his new Red Bull team mate Australian Daniel Ricciardo has out-performed him.
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