Kimi Raikkonen led a Ferrari one-two to outpace Mercedes in second practice at the Chinese GP and suggest F1's world champions could have a fight on their hands this weekend.
Having warned that their Italian rivals have not yet shown their full potential in 2016, Mercedes lost out to Ferrari on the supersoft tyre runs mid-way through the session by two tenths of a second.
After matching his best finish for three years in Bahrain, Raikkonen ended the opening day in Shanghai on top with his 1:36.896 time one tenth of a second quicker than team-mate Sebastian Vettel.
Mercedes completed their qualy-style, supersoft tyre runs slightly earlier and Nico Rosberg and Lewis Hamilton wound up third and fourth respectively. Hamilton, who will start Sunday's race no higher than sixth owing to a gearbox change penalty, was two tenths adrift of his championship-leading team-mate, who had earlier topped P1.
"It's tantalising to see if Ferrari can actually get up there in terms of performance," said Sky Sports F1's Ted Kravitz.
"But still the reliability question hangs over them. As we understand it, they do understand the reasons for Sebastian Vettel's failure in Bahrain and they are sure that particular problem won't happen again."
Vettel, who did not take the start in Bahrain after a valve failure, expressed hope Ferrari could move to within striking range of Mercedes in qualifying: "It depends what the others are doing, but I think if we get everything right we could be closer. But then it is only Friday today so I wouldn't stress, I wouldn't put too much importance in the times today."
In keeping with the Mercedes-Ferrari dominated start to the season, the pack were a long way adrift around the demanding Shanghai circuit - although Red Bull strengthened their claim to be the grid's third-quickest team by finishing fifth with Daniel Ricciardo.
Max Verstappen was close behind in sixth for Toro Rosso, three tenshs clear of the first Force India of Nico Hulkenberg and the second Toro Rosso of Carlos Sainz.
Having been given the go-ahead to complete the race weekend by the FIA's doctors, Fernando Alonso ended his first day back in McLaren's car with the 11th-fastest time as the team's two former world champions finished just outside the top 10.
"There is more pace to come which is encouraging," team boss Eric Boullier told Sky F1. "There is a long straight here which is not in our favour, but we hope [for points]."
Williams, Friday's perennial low-key runners, were 10th with Valtteri Bottas but the dramatic events of the earlier session meant there was more focus on them than usual on day one.
Having traced the double rear-tyre failure on Felipe Massa's FW38 in P1 to cracked wheel rims, the Brazilian returned to set the 14th-quickest time in P2 and afterwards said: "We made a modification and we understood the problem. If we didn't know the problem it would have an effect. but the team understand so I trust them.
"We can have a completely different day tomorrow. We need to be ready for everything. We can have a lot of different sessions."
After his own mechanical-triggered puncture in P1, Renault's Kevin Magnussen was not so fortunate in the afternoon with the Dane sidelined for the whole session while mechanics worked to repair the RS16's rear suspension.
Practice Two timesheet
1. Kimi Raikkonen, Ferrari, 1:36.896
2. Sebastian Vettel, Ferrari, 1:37.005
3. Nico Rosberg, Mercedes, 1:37.133
4. Lewis Hamilton, Mercedes, 1:37.329
5. Daniel Ricciardo, Red Bull, 1:38.143
6. Max Verstappen, Toro Rosso, 1:38.268
7. Nico Hulkenberg, Force India. 1:38.527
8. Carlos Sainz, Toro Rosso, 1:38.542
9. Sergio Perez, Force India, 1:38.569
10. Valtteri Bottas, Williams, 1:38.723
11. Fernando Alonso, McLaren, 1:38.728
12. Jenson Button, McLaren, 1:38.828
13. Daniil Kvyat, Red Bull, 1:39.178
14. Felipe Massa, Williams, 1:39.214
15. Jolyon Palmer, Renault, 1:39.774
16. Romain Grosjean, Haas, 1:39.890
17. Pascal Wehrlein, Manor, 1:39.941
18. Marcus Ericsson, Sauber, 1:39.979
19. Rio Haryanto, Manor, 1:40.550
20. Felipe Nasr, Sauber, 1:41.066
21. Esteban Gutierrez, Haas, 1:42.954
22. Kevin Magnussen, Renault, No time
Watch the Chinese GP weekend - LIVE ONLY on Sky Sports F1. The race starts at 7am on Sunday, with build-up underway from 5.30am. Or watch without a contract for £6.99 on NOW TV.