After Jenson Button finished the Jerez test with the best time, Fernando Alonso warned that Ferrari is yet to show its "muscles".
The Spaniard's best time weighed up 1.5 seconds behind the McLaren, but he told AS newspaper "there is no reason to be pessimistic" with another test to run in Barcelona next week.
A double world champion with Renault in 2005 and 2006, he warned that the F10 is "already the best car I've ever had".
"Red Bull, McLaren and Renault have gone quick and shown their cards," said the 28-year-old, "while we are still hiding ours.
"If I was with another team I would be looking at Ferrari, because everything so far is working very well."
Alonso suggested that one area for Ferrari to turn to is running with low fuel for qualifying simulations.
"This is where Red Bull and McLaren are showing very fast, and is certainly the basis for trying to improve the performance of the F10 in the next test of Catalonia (Barcelona)," he said.
Barcelona will also be a key indicator of the pecking order, because teams will run in their near-definitive packages for the looming Bahrain opener.