Toro Rosso boss Franz Tost says several disappointing upgrades have left the Faenza team struggling and under pressure from Sauber to maintain its eight place in the Constructors' standings.
The junior bull squad's 2018 campaign has been marked by the inconsistent performance, in both qualifying and race trim, of its STR13 chassis.
Recent updates failed to deliver the expected step forward, and although Toro Rosso has outscored Sauber in the last two races, Tost fears his drivers will be challenged to keep their direct rivals at bay.
"We haven't achieved our goals with the upgrades, we expected more from it," Tost told Motorsport.com.
"The midfield is very close, it's [covered by] hundredths of seconds. If you can't improve then it's difficult to defend the position, and currently we are behind our expectations.
"I hope for the second half of the season we can improve our performance. It's difficult, yes. Sauber, Haas made big steps forward, also thanks to their fantastic Ferrari power unit.
"Therefore it has become difficult for us to be within the first 10."
Unfortunately, Toro Rosso's quandary could get worse before it gets better, with Pierre Gasly not expecting the team to shine at Spa and Monza, two power tracks where the Honda-powered STR13 is expected to struggle.
"I think there are still a couple of things we don't understand, why sometimes we perform a lot more than we expect and some other times we struggle a lot more than we think," said the Frenchman, addressing the team's inconsistent performance.
"Of course we know that there are tracks where we're going to perform a lot more, I think Spa and Monza will be quite tough for us, but Singapore is another one where we need to focus [to get a strong result]."
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