The Andretti driver was only able to finish 11th, having struggled for pace throughout the 28-lap race at the Shanghai International Circuit after starting down in 14th.
With Dennis only able to take fifth place from the opening race of the weekend a day earlier, it means the Briton is now 55 points behind championship leader Nick Cassidy with four races remaining.
“He [Cassidy] is way too far ahead now, for sure I’m out of the championship now,” Dennis told Motorsport.com.
“I just want to try and fix our pace for next year, obviously I want to win races this year now but right now we’re so slow in qualifying and I wasn’t quick in the race today. So we need to fix our issues for next year.
“I’m a bit disappointed the championship is over, but they’ve just done a better job than us. We haven’t been quick enough.”
Dennis has only taken one win this season, the first race in Diriyah, and has not stood on the podium in the last five races, which has left him fifth in the standings.
And while team-mate Norman Nato claimed his first rostrum for Andretti on Sunday, Dennis was left struggling down the order with handling problems.
“I was just slow, I was slow from lap one to lap 28, I just had no pace whatsoever, even in Attack Mode I was still going backwards,” he added.
“I was really struggling with the car, just had oversteer like no other, which is the complete opposite to yesterday.
“We just need to understand why really, that’s the biggest thing. The best I was ever going to get was maybe eighth even if I was fast so we just take it on the chin.”
Cassidy’s nearest title challenger remains Pascal Wehrlein, but the Porsche driver also failed to score in the second China race, having suffered a left-rear puncture after contact with Sam Bird.
It means the German is now 25 points – or a race win – behind Cassidy with just two double-headers in Portland and London remaining.
“In the end, the problem was our qualifying pace. We found a mechanical issue on the car after qualifying which we fixed for the race, very likely from the race yesterday," said Wehrlein.
“That meant we started from 13th and the whole day was compromised by the puncture and qualifying.
“It’s not a difficult situation [the championship], it’s still a very good situation. Four more races to go, we know that if we put our stuff together then we are competitive, no matter which track.
“Obviously we need to make sure that we maximise what we have.”
