The Silver Arrows did not venture out for a long time at the start of the session, with McLaren initially setting the pace. When they did emerge Rosberg duly went fastest with 1m 48.575s on the soft Pirelli tyre, with the Red Bull's snapping at his heels, half a second down. Hamilton, meanwhile, was struggling, having to catch a big slide at Turn 5 and lurking eight-tenths of a second down on his team mate.
When the field switched to the red-marked supersoft tyres Rosberg again set the pace, finding around one second to post 1m 47.488s, but surprisingly that time was then beaten first by Alonso, with 1m 47.299s, and then Ricciardo, with 1m 47.350s.
The surprises did not stop there: while Hamilton's effort on the supersoft drew him closer to Rosberg - he posted a 1m 47.738s - he ended up sixth, behind a superb lap from Toro Rosso's Jean-Eric Vergne on 1m 47.693s for Toro Rosso and Sebastian Vettel in the second Red Bull on 1m 47.711s. Felipe Massa was fractions of a second further down the road, managing 1m 47.909s in an improved Williams.
Perhaps Mercedes were therefore sandbagging, or preserving rubber for what promises to be an incredibly tight qualifying later this evening - or perhaps the session represented the first of the curveballs that Ricciardo predicted yesterday...
Valtteri Bottas wasn't as quick as his Williams team mate, but was eighth on 1m 48.205s in the second Williams, with Kimi Raikkonen right on his tail with 1m 48.226s in the second Ferrari. Esteban Gutierrez meanwhile was a surprise 10th, indicating improvements for Sauber with 1m 48.422s - less than three hundredths of a second ahead of Nico Hulkenberg's Force India, on 1m 48.450s.
After their early pacesetting the McLarens fell back, with Kevin Magnussen only 12th on 1m 48.577s and Jenson Button next on 1m 48.599s, while Daniil Kvyat survived a nasty scare under braking to take 14th on 1m 48.637s.
Sergio Perez had a disappointing session, his 1m 49.078s leaving his Force India only 15th in the line-up ahead of Adrian Sutil's Sauber on 1m 49.115s and Romain Grosjean's Lotus on 1m 49.485s. Pastor Maldonado, with a new chassis following his crash on Friday, kept out of the walls to lap the Marina Bay circuit in 1m 50.149s, but both he and team mate Grosjean were having to work very hard en route to finishing 17th and 18th respectively.
Jules Bianchi was again in great form for Marussia, lapping his MR03 in 1m 50.376s to stay ahead of Kamui Kobayashi, who squeezed 1m 50.939s out of his Caterham. Their respective team mates Max Chilton and Marcus Ericsson rounded out the timesheets, 1m 51.221s to 1m 51.598s.
The scene is therefore brilliantly poised ahead of qualifying, with no clear pattern yet to emerge -throwing up the possibility of some significant surprises at the sharp end.