The Grove-based squad was stuck at the back of the field throughout 2019, unable to compete against rival teams.
It marked a continuous downward spiral from 2018 when Williams also ended the championship in last place - albeit with stronger performances throughout the season.
In 2019, it scored one point at the hands of Robert Kubica in Germany, while Russell ended the year as the only driver not to hold a top ten finish.
For the 2020 season, Russell will be joined at the team by Nicholas Latifi, and he expects there to be an improvement as the team has discovered the root of its issues.
"We had to look at the basic structure of the car," Russell told Auto Motor und Sport. "There were aspects that were fundamentally wrong.
"Something was wrong with the aero. The downforce was not constant and there were also times in 2018 when Lance and Sergey crashed out of nowhere.
"We had to start from scratch when it comes to the downforce but since then we've been on a good path."
Russell added that the car will be an "evolution" from the 2019 design, but is convinced that strides forward will be made.
"We committed to a strategy at the end of 2018 and it took time to rebuild the foundation of the car," the 21-year-old said.
"We didn't see any progress in the wind tunnel for a long time but the trend is different now.
"We will definitely be better in 2020."
However, despite the confidence that the team will make improvements, Russell knows that 2020 will once again be a tough year for Williams.
"I expect that we will fight the others more often, but realistically it will be another difficult season for us," Russell admitted.
"McLaren took a big step from 2018 to 2019, and if we can do a similar step and Haas, Racing Point and Toro Rosso do not progress, we will be in a very strong position. If they all find a second or two, we'll stay where we are."
Circuit de Catalunya - Winter testing
Bahrain International Circuit - Winter testing
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