Lewis Hamilton dominates European Grand Prix practice in Azerbaijan
Lewis Hamilton set a stunning pace to head second practice at the inaugural European Grand Prix in Azerbaijan from team-mate Nico Rosberg.
Hamilton was 0.69 seconds clear as Mercedes appeared in a race of their own, with Ferrari off the pace.
Sebastian Vettel was only eighth - 1.9secs slower than Hamilton - as Force India's Sergio Perez took third.
The session was run after quick repair work to kerbs, which had been cutting tyres in the first session.
There were no immediate reports of further problems after the kerbs were welded to prevent sharp edges coming loose, but further repairs are inevitable overnight before qualifying on Saturday.
Vettel was one of a number of drivers to explore the run-off areas at the demanding, new 3.7-mile street circuit in the nation's capital Baku, which features F1's longest straight.
But there were no repeats of the crashes suffered by Red Bull's Daniel Ricciardo and Toro Rosso's Carlos Sainz in the first session.
A number of teams were out of their usual positions - in addition to Vettel, his Ferrari team-mate Kimi Raikkonen was down in 11th and the Red Bulls of Max Verstappen and Daniel Ricciardo only seventh and 10th.
Williams' Valtteri Bottas was fourth, ahead of Force India's Nico Hulkenberg and Carlos Sainz's Toro Rosso.
McLaren's Jenson Button was ninth, two places ahead of team-mate Fernando Alonso.
Hamilton's pace advantage over Rosberg was just as big on the long race-simulation runs - Hamilton was 0.7secs on average quicker than his team-mate after four laps, before Rosberg's car failed and he had to stop out on track with a lack of drive.
After running Mercedes close in Canada last weekend, Ferrari were struggling again in Baku, their recurring problem of struggling to get the tyres up to the correct operating temperature biting them again.
And although Rosberg and Button raised concerns about the lack of run-off areas at some corners on Thursday, the safety features seemed to work well through both sessions.
Haas's Esteban Gutierrez explored them the most regularity - he suffered multiple off-track moments through the session, and Raikkonen's session ended five minutes early when his rear wheels locked and he slid into an escape road and had to park up.
Vettel also hit trouble at the end of the session, being ordered to slow down and return to the pits with a minute to go.
The team told Vettel that he had suffered an issue with his MGU-K - the device that recovers energy from the rear axle in these hybrid cars - as had Raikkonen.
"The speed wasn't quite there, so we need to have a look and see to be in better position tomorrow," Vettel said.
"It's not nice, it's not good news [to be two seconds off the pace]. Some of it we can explain, and other bits we need to wait and see what tomorrow brings. It's been a tricky day for us, not entirely clean when you look a raw pace."
The drivers will discuss the issue of track safety, among other matters, with FIA F1 director Charlie Whiting in their briefing on Friday afternoon.