Part of Sebastian Vettel's struggle so far in 2014 is "fatigue" after four consecutive world championship triumphs.
"You have to congratulate Ricciardo," Austrian Berger told APA news agency. "He does an excellent job, and so consistently right from the start which is not something expected."
Behind Ricciardo's shining performances, however, is a Vettel-factor, Berger argued.
"We cannot forget that Vettel has won the championship four times; after four times with this pressure there is a certain fatigue that eventually makes its presence felt.
"Four times you win everything," Berger explained, "and then you get a car that you know you cannot win the race with due to engine performance. This is very hard to deal with.
"Ricciardo is pleased about a podium but Vettel is not interested in third place; he wants to win the world championship," he added.
Red Bull and Vettel have upped their pace recently, but Berger doubts that Mercedes can be challenged this year -- and he points the finger mainly at Renault.
"Mercedes has had a great engine for years," he said, "but Red Bull has been able to compensate with Vettel and the aerodynamics.
"If Renault wants to catch up now, they will need to upgrade their budget and their people. We can't forget about the resources that are behind Mercedes," added Berger.
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