Sauber is now racing "on an equal footing" with Williams, according to the Swiss team's driver Marcus Ericsson.
Sauber was F1's slowest team in the past two years, but new management and a deal with Alfa Romeo has led to an uptick for 2018.
At the same time, Williams's form has collapsed.
"It looks like we can compete on an equal footing with Williams," Ericsson told Auto Motor und Sport.
"In the last two years, it was all about survival. Now it's time for results.
"In practice we are sometimes even with Force India or Toro Rosso, which is a huge step compared to the last two years."
The 2018 season continues this weekend in Baku.
