"And, two: the circumstances we raced under in Silverstone are definitely not we what we accept and should they repeat we definitely can't race."
German GP: First practice highlights from the Nurburgring
All but three of the drivers are represented by the Grand Prix Drivers' Association (GPDA), which put out a
statement on Thursday
saying they would "immediately withdraw" from this race in the event of further failures.
Chairman Pedro De La Rosa clarified on Friday morning that in the event of a failure the drivers would seek to establish what has happened with tyre supplier Pirelli before making any decision about how to proceed.
The three non-members are Lotus's Kimi Raikkonen, Force India's Adrian Sutil and Williams's Valtteri Bottas. Raikkonen and Bottas sad they would race regardless, while Sutil said he would "think about" joining any boycott should there be one.
Pirelli has supplied a new tyre using different materials in its construction to ensure there were no further problems.
The Mercedes cars were fastest by a second in the first session, with Hamilton ahead of team-mate Nico Rosberg. But Vettel turned the tables on them in the second, pipping Rosberg by 0.235 seconds.
Red Bull's Mark Webber was third in the second session, ahead of Lotus's Romain Grosjean and Kimi Raikkonen, and Ferrari's Fernando Alonso.
Hamilton was eighth fastest behind Ferrari's Felipe Massa.
McLaren's Jenson Button was ninth fastest ahead of Scot Paul Di Resta in the Force India.
Hamilton was 0.653secs behind Rosberg on one-off lap time but looked in good shape compared to his partner on the race-simulation runs later in the session.
Those laps, when the teams fill the cars up with fuel and prepare for the race, suggested that Ferrari were back to their usual form after
struggling at Silverstone.
Red Bull looked very strong on race pace while Mercedes were difficult to read - they are either in the region of half a second slower than the Red Bulls and Ferraris or were running a heavier fuel load to see how their tyres performed under the most challenging conditions.
Vettel said: "Mercedes were quick this morning. We couldn't have done that [time].
"This morning I wasn't entirely happy with the car. This afternoon I was in better shape. The long run I think was determined by traffic etc, but I'm quite happy with what we had in the afternoon."
"It's been OK, nothing special," said Hamilton. "The first session was pretty good and then we made some changes, but we've gone backwards. We'll make some other changes and we'll be fine I'm sure.
"At the start of the second practice session we got a little bit of imbalance in the car. But it's fixable."
Saturday 6 July: Third practice at 09:55 BST on BBC Radio 5 live sports extra plus live text and audio commentary online. Qualifying at 12:55 BST on BBC Radio 5 live sports extra plus live text commentary online. TV highlights of qualifying at 17:55 on BBC One
German Grand Prix, day three
Sunday 7 July: Race coverage at 12:30 BST on BBC Radio 5 live sports extra plus live text commentary online. TV highlights at 18:00 BST on BBC One
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