Carlos Sainz Jr says Honda's pace in Formula 1 qualifying at Suzuka was "worrying", but the Spaniard remains confident that Renault still has the upper hand on race pace.
The Toro Rossos of Brendon Hartley and Pierre Gasly both made Q3 at the Japanese Grand Prix, but in the race they slipped out of the top 10, while Sainz moved up to 10th to claim the final point, passing Gasly in the closing stages.
"In qualifying, it's difficult to say," said Sainz when asked if Honda is now ahead of Renault. "In the race definitely not, I don't think we're behind Honda on race pace.
"I managed to overtake both Honda cars in the race at Suzuka with better race pace than them, which was a good boost for us.
"But in qualifying they took a step which I think for us starts to be worrying. They took a step in qualifying that we didn't have, and that put them in Q3."
Sainz believes the engine has been the weakness that has enabled main rivals Haas and Force India to get ahead of Renault in the latter part of the season.
"We've taken the biggest hit from the engine side," he said. "We thought by Canada when we were clearly leading the midfield that the B Spec [engine] had been successful, which it was at the time.
"Up until Hockenheim we were happy enough with what we had, we thought we were going to the end of the year being a Q3 team and scoring points, but then those upgrades that Ferrari brought to Austria, to Hockenheim, to Hungary, we started to feel them a lot in places like Spa and Monza, and consequently places like Suzuka.
"So to compensate that big engine deficit we should have brought a lot more aero than what we brought, but it's mainly dominated by engine."
Renault has stated that its 2019 engine will be substantially revised, and Sainz - who will use it when he moves to McLaren - has been encouraged by what he's been told.
"That's the last official statement, it's going to be quite fresh," he said.
"From what I heard it's still going to deliver a step for Renault and McLaren next year.
"Thanks to that we should bring the whole pack together and be closer in top speeds to our competitors.
"We shouldn't [need to] start compromising the rear wing for these kinds of tracks, and thanks to that we should start to fight the Mercedes and Ferrari cars on the straights, not only in the corners.
"I'm always optimistic at this time. I can also see optimistic people inside Renault.
"But in winter testing, maybe my face changes. Up until then I prefer to be optimistic, and think that Renault are finding going to deliver an engine that is nearly at the level of Ferrari and Mercedes."