Nico Rosberg has said only luck prevented him from crashing into the wall after he suffered a spectacular 200mph tyre blowout in practice for the Belgian Grand Prix.
The 30-year-old German was approaching Blanchimont when he suffered a catastrophic right-rear tyre failure on his Mercedes.
Rosberg was a helpless passenger as his car was sent into a high-speed slide following the tyre explosion, but mercifully he avoided a heavy impact with the wall.
"It was quite a shock because I didn't expect it," Rosberg, whose wife Vivian is due to give birth to their first child next week, said.
"It was not a nice experience at nearly 200mph to suddenly lose control. Something failed on the right rear.
"Unfortunately it wasn't down to my skill, it was just luck that I stayed away from the wall.
"Of course it is a little bit of a dent in the confidence. I didn't feel anything before - there was no warning at all - so there is going to be a big analysis."
Rosberg refused to speculate on the cause of the failure while Pirelli - the sport's sole tyre supplier - vowed to carry out a full investigation into the harrowing incident.
Rosberg can, however, be heartened by a strong display on Formula One's return to action in the idyllic surroundings of this historic Spa-Francorchamps circuit.
He set the pace in both practice sessions and his fastest lap of one minute 49.385 seconds in the afternoon was three tenths quicker than Hamilton, who leads Rosberg by 21 points in the drivers' standings.
Daniel Ricciardo and Daniil Kvyat were next up for Red Bull with Kimi Raikkonen, who has extended his stay with Ferrari into 2016, fifth fastest.
Hamilton said: "Nico is very quick so I have some work to do tonight to see where I can find some time.
"Ferrari and Red Bull also look closer here than they have at past weekends, so it'll be a close run thing.
"What happened towards the end with Nico obviously wasn't good but luckily he's unhurt.
"I have no concerns, though. You have to have trust in the car and in the tyres to go out there and push, so it's not something you think about."
McLaren's season has long since gone from bad to worse, but here they were dealt a fresh twist - a record combined 55-place grid drop for Sunday's race.
Button was hit with a 25-place grid penalty after taking on his seventh engine of the season and for additional component changes.
Alonso, who finished fifth at the Hungarian Grand Prix, will serve an even greater penalty after having a raft of new parts fitted to what is also his seventh different Honda engine of the campaign.
The pair will be demoted to the back of the grid, but, with only 20 cars entered into Sunday's race, will not face any subsequent punishments. A ruling was put in place following their accumulative 50-place grid penalty in Austria earlier this season.
McLaren have brought a new-specification Honda engine to Spa - a track where power is integral - but the end result was the same on Friday. Button and Alonso ended the second session ahead of only the Manor cars of Will Stevens and Roberto Merhi in 17th and 18th.