Mercedes duo Lewis Hamilton and Nico Rosberg say they face a revived challenge from Ferrari at this weekend's Chinese Grand Prix.
Ferrari have struggled so far this season, but Fernando Alonso was second fastest in Friday practice, splitting Hamilton and Rosberg.
Hamilton welcomed a resurgent Ferrari and also highlighted the pace of the Williams, adding: "We want a fight."
Rosberg added: "We still look quickest but the competitors are pushing."
Ferrari toiled in the last race in Bahrain, where Alonso and team-mate Kimi Raikkonen could manage only ninth and 10th respectively.
The poor performance led to the resignation of team principal Stefano Domenicali, who has been replaced by Marco Mattiacci, former head of Ferrari's road-car division in North America.
But the different characteristics of the Shanghai track are playing to Ferrari's strengths in fast corners. The Italian team have also found an improvement in straight-line speed, a weakness in the first three races.
Alonso said: "We have brought some small updates here, which we already tried at the Bahrain test.
"Everything worked well and that's good news. Leaving aside the performance of the others, I am happy with what we have done today."
The race-simulation runs in the last part of the session, when the teams run on high fuel loads in preparation for the grand prix, suggested that Ferrari are still a second a lap slower than the Mercedes.
Alonso's pace was similar to that of Red Bull, Force India and Williams, but McLaren appeared to be some way adrift of that group.
Rosberg was happy with the performance of his car, but Hamilton said he still had improvements to make.
The Englishman lost about 40 minutes of running with a suspension problem that caused him to stop first practice early and get out late in second practice.