Lewis Hamilton declared his Chinese Grand Prix a "disaster" on Sunday and said Mercedes would find it tough to repeat their F1 championship victory unless they can sort out issues with a car he feels uncomfortable in.
Daniel Ricciardo of Red Bull won, ahead of Hamilton's Mercedes teammate Valtteri Bottas in second.
"Saturday and Sunday felt like a disaster from my side," he said.
"It is clear from this weekend we are not the quickest. We have lost performance since Melbourne and maybe even more so this weekend." "We are the second or third-fastest team at the moment, so we have some improving to do, but that's not impossible."
Germany's Vettel and Ferrari put champions Mercedes on the defensive with a season-opening victory in Australia, followed last weekend by a close win in Bahrain.
The result in Shanghai could mean more questions asked of Mercedes, even though they maintain a one-point advantage in the constructors' standings.
"Who knows what the season holds? If it continues the way it is, it's going to be very tough to win," Hamilton said.
"But if there is an opportunity, and we were to finish on top, it would mean even more because it's an even tougher season than before." Hamilton said he "struggled" with his car in Shanghai. "Rather than blame it on strategy or anything else, I was uncomfortable in the car this weekend." Said Bottas:
"It's very close between Red Bull, Ferrari and us, so we need to make sure we keep developing our car."
"Nearly winning both last weekend and this weekend is the worst feeling -- but it makes both me and the entire team even hungrier. Thankfully there are a lot more races to go.