The incident happened just over an hour into the 90 minute session and left an unhurt Rosberg stranded on the approach to Blanchimont, having luckily avoided the barriers after his right-rear Pirelli let go.
Marcus Ericsson was less fortunate. The Sauber driver brought the red flags out again in the session’s closing stages as he lost control at Pouhon, swiping the front-left corner of his car against the wall. The Swede emerged unscathed.
Third in the times behind the two Mercedes was Red Bull’s Daniel Ricciardo, seven-tenths down on Rosberg’s best, followed by team mate Daniil Kvyat. Next up was the Ferrari of Kimi Raikkonen, the Force India of Nico Hulkenberg, and the Lotus of Romain Grosjean.
Completing the top ten were Ericsson, Sauber team mate Felipe Nasr, and Sebastian Vettel in the second Ferrari.
It was Rosberg who had set the pace on the medium tyres, and he maintained his momentum after the field switched onto the soft tyre, cutting down to a 1m 49.385s. Hamilton was in his shadow, but was half a second down on the mediums and three tenths down on the softs, posting a best of 1m 49.687s. He was also troubled at points by poor downshifts.
Rosberg then switched onto a longer run on the softs, and had completed 11 laps when, barrelling into Blanchimont, his right rear gave way.
Watch: Rosberg's big moment at Blanchimont
Once again the two Red Bulls were in play, suggesting that they have indeed made a lot of progress with the RB11's chassis and aerodynamics. Ricciardo was third with 1m 50.136s, while Kvyat backed him solidly with 1m 50.399s. Later, however, the Australian voiced suspicions that his engine felt like it was about to break.
Ferrari's best runner was Raikkonen on 1m 50.461s, a time later matched by Hulkenberg, while Grosjean ended up seventh on 1m 50.489s. The Frenchman, however, is being investigated for impeding Hulkenberg early in the session. Right at the end, Grosjean rolled to a halt and was stranded on track with power loss.
Ericsson had earlier been temporarily fastest when he switched to the softs, before Rosberg restored the status quo. He held on to an encouraging eighth with 1m 50.709s ahead of team mate Nasr on 1m 50.928s, both revelling in the performance of the upgraded Ferrari power units which are to the specification Ferrari themselves had several races ago. The Brazilian had a scare when Kvyat was released right into his path in the pits, another incident the stewards are investigating.
Vettel finished the session a subdued 10th on 1m 50.940s, with Sergio Perez in the second Force India right behind on 1m 50.971s. The Mexican went fastest in sector one for a while, but had to go round Felipe Massa's Williams heading down to Pouhon, which didn't help his lap.
Toro Rosso weren't as fast as they had been in the morning, relatively speaking, with Carlos Sainz this time easing ahead of Max Verstappen, 1m 51.037s to 1m 51.117s. Valtteri Bottas was next on 1m 51.250s after a rear wing endplate was changed, and Pastor Maldonado wheeled his rebuilt Lotus round in 1m 51.317s for 15th ahead of Massa on 1m 51.588s. One has to assume that Williams were majoring on race work, as they did in Hungary.
It was another disappointing outing for McLaren; both drivers were using the Mk3 version of the Japanese engine, but Button's best of 1m 51.854s was well away from where Honda had expected to be. Alonso was behind him on 1m 52.570s, having spoiled a faster lap by locking up badly going into the Bus Stop chicane. With all of their various component replacements and engine changes, the two of them face respective grid penalties of 25 and 30 places.
Will Stevens continued to lead the inter-team Marussia fight with 1m 54.065s, but Roberto Merhi was on his tail with 1m 54.253s.