Carlos Sainz Jr feels Toro Rosso has fallen behind Red Bull on the development curve and anticipates a tricky weekend when the rest of his rivals step up the pace.

The Spaniard pinpointed the Red Bulls as the standout performers during Friday's free practice sessions and despite ending the day sixth fastest he says there is plenty of work to be done if he is to convert a promising start into points.

"It has been a positive day but unfortunately we know for qualification and the race those with Mercedes engines will make a step forward," Sainz said. "We need to keep an eye on Force India because they looked strong even though they didn't run in FP2.

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"We also need to find out why Red Bull are suddenly so quick compared to us and see if we are missing something that they have found but overall we are where expected to be, maybe even a bit further ahead. But we expect a tougher day on Saturday."

Sainz explained Toro Rosso is still losing out in similar areas on the slower speed circuits and that is where Daniel Ricciardo and Daniil Kvyat have impressed.

"Red Bull look to have a bit more straight line speed than us and like Monaco a little bit more mechanical grip which we are not able to find," he said. "In higher speed corners we are competitive in fifth, sixth and seventh gear corners but as soon as we go to second and third gear they look to have a bit more and they are stronger."