Ferrari may be poised in China to secure its first victory of 2016.
The red cars were first and second in Friday practice at the Shanghai circuit, and fast not only over a single lap but also in the crucial 'long runs'.
Mercedes, whose Nico Rosberg and Lewis Hamilton have won every grand prix since last September, say they are taking notice.
"Ferrari is closer," a team engineer told Auto Motor und Sport.
"On a lap it is now three tenths rather than six. And in race trim they are a real challenge."
World champion Lewis Hamilton told Sky Italia: "It seems that we're going to have a bigger fight than we ever have, so it should be good for the fans."
It could also be perfect timing for the Maranello team, whose demanding president Sergio Marchionne is planning to attend the Chinese grand prix on Sunday, according to Italy's Tuttosport.
"We promise to earn more (money) and win in formula one," the Italian-Canadian said after a shareholders meeting in Amsterdam.
"These things are sacred and non-negotiable," Marchionne insisted.
Team boss Maurizio Arrivabene, however, moved to ease some of that pressure by describing Mercedes as "the strongest team I have seen in the last ten years".
But former F1 driver David Coulthard told the Telegraph: "Ferrari don't really have any excuses. They have done their restructuring, have all the budget in the world, and still don't quite seem to be there.
"Now is their chance to prove us wrong," he said.