The German made an error at Mirabeau on his final run, sliding up the escape road and bringing out the yellow flags. Everyone behind Rosberg - including Hamilton who had just set a personal best first sector - were thus unable to improve.
After the session the stewards announced that the incident involving Rosberg was to be investigated, but after a short period of deliberation they could find no evidence that the German had committed any offence.
Behind the duelling Silver Arrows, the Red Bulls of Daniel Ricciardo and Sebastian Vettel took both slots on the second row, whilst the Ferraris of Fernando Alonso and Kimi Raikkonen locked out the third.
Such are the stakes at Monaco that everyone bar Mercedes and Red Bull opted to run their supersoft tyres from the start of Q1, which complicated the issue by putting some unfamiliar faces in high places. Not least of these was Jean-Eric Vergne, who eventually beat the Mercedes to fastest time in 1m 17.557s as Rosberg edged Hamilton by just over a tenth.
Kvyat was the first man to clobber anything seriously this weekend, when he got into a tank slapper under braking for the chicane and damaged his Toro Rosso’s nose against the inner wall. He got going, however, and redeemed himself by scraping into Q2 in 16th on 1m 18.616s.
That left the Saubers out in the cold, with Esteban Gutierrez beating Adrian Sutil by four thousandths of a second, 1m 18.741s to 1m 18.745s.
Jules Bianchi was unable to match his morning time and took 19th ahead of Marussia team mate Max Chilton, 1m 19.332s to 1m 19.928s. Kamui Kobayashi was 21st on 1m 20.133s, leaving team mate Marcus Ericsson 22nd on 1m 21.732s. The Swede caused the other upset of Q1 when he dived past a slowing Felipe Massa at Mirabeau, only to nudge the Williams driver into the wall as he overshot the bend. The Brazilian was not amused - his session was over despite setting the 10th fastest time - but at least he could take some satisfaction from the fact that the stewards announced an investigation into the incident.
Also are under investigation are Sergio Perez and Esteban Gutierrez and Pastor Maldonado and Kvyat for alleged impeding offences.
As Hamilton beat Rosberg to fastest time by a tenth in Q2, Sergio Perez squeezed into Q3 at Force India team mate Nico Hulkenberg’s expense. The German was 11th on 1m 17.846s from Jenson Button, on 1m 17.988s. It’s worth pointing out that nine-tenths covered third down to 12th!
Behind the McLaren, Williams’ miserable day continued with Valtteri Bottas only 13th on 1m 18.082s, as the Lotuses of Romain Grosjean and Maldonado also failed to make the cut with 1m 18.196s and 1m 18.356s.
Thus came the big showdown in Q3. Well, almost.
Rosberg snuck back ahead of Hamilton with 1m 15.989s to 1m 16.048s, as Ricciardo aced Vettel 1m 16.384s to 1m 16.547s. Behind them Alonso recorded 1m 16.686s and Raikkonen 1m 17.389s.
Further back, Jean-Eric Vergne capped his excellent form thus far in the Principality by taking seventh for Toro Rosso in 1m 17.540s, ahead of the two rookies. Kevin Magnussen got a great eighth place for McLaren with 1m 17.555s as Daniil Kvyat made a brilliant recovery from his Q1 crash to take ninth on 1m 18.090s in the second STR9. Perez rounded out the top ten with 1m 18.327s.
Then came the second runs. Ricciardo failed to improve after a big slide, then Rosberg was already down on his sector one time from his first lap when he locked his right-front wheel braking for Mirabeau and slid up the escape road. That seemed to leave the way clear for Hamilton and Vettel to attack, but the yellow flags were still showing at Mirabeau when Hamilton arrived and though the Briton had matched Rosberg’s first-run Sector 1 time - and was by his estimate a couple of tenths up - he had no choice but to back off.
The fight for pole thus ended in an anti-climax, and left one Mercedes driver smiling sheepishly and the other looking frustrated.