Lewis Hamilton said Mercedes still needs to make "big changes" if it is to challenge Red Bull for wins this season.
Hamilton is set to leave the team at the end of the year for Ferrari, but has pledged to end his Mercedes career on a high.
- Unlapped: How to listen or watch ESPN's F1 show
At the opening two rounds of the season, he finished seventh in Bahrain and ninth in Saudi Arabia, leaving him ninth in the drivers' standings and 43 points off runaway championship leader Max Verstappen.
Mercedes made significant changes to its car over the winter after two difficult years under the current technical regulations, but Hamilton believes it has not gone far enough.
"We have definitely got to make some big changes," he told Sky Sports. "We haven't made big enough changes, perhaps.
"If you look at the three teams ahead of us, they still have a different concept to where we are in some areas. So we have got some performance to add, for sure."
After finishing ninth in Saudi Arabia, Hamilton believes Mercedes' main issue is a lack of performance in high-speed corners compared to main rivals Red Bull, Ferrari, McLaren and Aston Martin.
"The car is good in the low speed and not so bad in the medium, but in the high speed we are miles off," he said.
"It was like I was in a different category when I was with the other guys around me. It's frustrating for sure to be three years in a row almost in the same position.
"It's definitely tough, but we'll get our heads down, keep working away, and I know everyone back in the factory is pushing as hard as they can."