Verstappen: Red Bull's F1 form key to make me stay long-term

The Dutchman has endured a frustrating campaign with Red Bull this year, with reliability problems blighting his season.

Red Bull has Verstappen under contract until the end of 2019, and team boss Christian Horner is confident Verstappen will not be tempted away by Ferrari or Mercedes.

Speaking at last weekend's Zandvoort DTM race, Verstappen said his commitment to Red Bull in the long-term depends entirely on how competitive its car is.

"I've always said that there's no reason to leave when I have a competitive car," said Verstappen.

"At the moment we are not there where I want to be, but that doesn't mean you have to give up straight away.

"You have to work hard.

"But after two, three years without improving, that would be a different story.

"We are working hard to improve but after next year we will see what happens."

Verstappen is aware that Red Bull's form this year has not been helped by Renault still having a small power deficit to Ferrari and Mercedes, which is why he thinks it hard to predict how his team will go at high-speed venues like Spa and Monza.

"Every race weekend is a bit different and that surprises me to be honest," he said. "On tracks with not too many straights, we are reasonably close, around three tenths more or less.

"When there's a few straights then we are a bit further away, so I don't know what to expect for the upcoming races.

"It will be quite tough to compete against them.

"Between Ferrari and Mercedes, it changes every weekend.

"I expect Mercedes to be very strong at Spa, because in general their power unit is still the best in the field, but it's very close."

Verstappen says that key to Red Bull moving forward over the remainder of this year will be down to chassis improvements, with Renault not planning any major update.

"We keep developing, but every team does," he said.

"So we have to make sure we do it a bit faster and of course better.

"On the engine side there's not much coming compared to the other teams, so we have to make sure our car is going in the right direction."