A fearful Lewis Hamilton insists Mercedes have "big problems" with their car after the championship leader failed to match Max Verstappen's race-winning pace at the Malaysia GP.
And Hamilton has also warned the Silver Arrows, now 118 points ahead of their rivals in the constructors' standings, will be cut adrift from Ferrari and Red Bull in 2018 if they do not rectify their faults.
Hamilton's frank admissions come despite the fact his second place in Malaysia came off the back of a three-race winning streak, while he has also extended his title advantage over Sebastian Vettel to 34 points with just five Grands Prix remaining.
"We've done an exceptional job with what we have," the downbeat three-time world champion told Sky F1. "There are some really big problems which I can't really explain to you.
"But we really need to make sure we rectify them for next year's cars if we really want to fight both Ferrari and Red Bull when they step up their game."
After a dominant one-two in Italy, team boss Toto Wolff admitted the pace drop-off in Singapore - where Hamilton won after starting fifth - and Malaysia was "worrisome" for the world champions.
Wolff also believes that without Ferrari's reliability struggles, with Vettel starting from the back on Sunday and Kimi Raikkonen not at all, Mercedes' pace would only have been good enough for fifth.
Vettel finished 19 seconds ahead of Valtteri Bottas and closed to within 10 seconds of Hamilton in the closing stages before settling for fourth.
"There is a lot of work for us to do," explained Hamilton. "But there's nothing we can do, it's the way the car is.
"People have been saying all year long that we've got the best car. It's a fact at some races that the car has turned out to be better but overall, globally, I think it's worked out that we haven't."
Mercedes are confident of regrouping ahead of a Japanese GP which should suit their W08's package, but Hamilton was still ruing the missed opportunity in Malaysia after Vettel's storming comeback drive.
"Considering Vettel started last, it's not really that great to be honest," he added. "We should have won today but we didn't have the pace.
"I don't know which one of these next races are going to be good for us but we'll do everything we can to try and stay ahead."
Lewis didn't want to 'risk' fighting Max
One of the more entertaining, and eventually defining battles on the day was between Hamilton and Verstappen, with the Dutchman overtaking the Mercedes into Turn One on Lap 4 before maintaining a healthy gap.
"I could have closed that door - but I didn't want to risk anything," Hamilton told Verstappen in the cool-down room, after the Red Bull driver eased to a second career F1 victory and his first of 2017.
And Hamilton told Sky F1's Craig Slater he had a fourth championship on his mind as the charging Red Bull approached.
"I really had to make a decision when Max was closing me down," he added. "I had to make the decision to not fight him and risk him coming into me because you know he's going to give it everything whereas I had everything to lose. I didn't make it very hard for him.
"At the end of the second stint it was looking a little bit better for us but the car was still having big problems at certain corners and I think Max was just managing the pace. I don't think we had the pace."