Button's latest time sees him jump above Magnussen into P1.
Ricciardo's first proper lap sees him leap to the top of the timing screens by some margin, a 1:50.762 some seven tenths quicker than anyone else. And the Australian's lap looked quite scruffy.
@LD: G'day. Renault said the engine was "at something like half-life" and were surprised it failed so early. His current engine is the one used in Hungary. A penalty later this season now looks inevitable.
Vettel slots in behind Ricciardo, who has improved on his first time to 1:49.588.
Remember the tyres get quicker and quicker so shouldn't compare runs when drivers have done different amount of laps.
@LD: Haha - had to be done I'm afraid!
@TomJ: Of course, while there is still a mathematical change.
Still yet to see timed laps from Hamilton or Rosberg, who are both in the garage, as are the Ferrari and Williams drivers.
@PeterM: Welcome Peter. Everyone on the soft tyre currently. We'll have some super-soft tyres at the end of this hour-long session when the track is a bit more rubbered in.
Looks like a real lack of grip out there at the moment. Lots of lock ups early on in this session.
Vergne gets very very close to the wall in Turn 21. Drivers really pushing the limits of the track in this session, which is great to see.
@LD: Yep. it's our little secret!
Vergne and Kvyat, impressively, third and fourth currently, though we haven't seen Williams, Ferrari or Mercedes set a time.
@Kristiyan: It's always hard to say without every radio message being broadcast, but Ricciardo seemed to be struggling with his rear tyres during the session. Hamilton had a few little gripes with his tyres too.
@grahamg: I think he's been hard done by certainly. Certainly deserves another chance at a more competitive team.
Rosberg and Hamilton are both out on track now, so let's see how they compare to Red Bull.
Excellent - Perez's engineer comes on the radio and says "OK, Sector One... Oh, not allowed."
Sounds like Caterham's engineer did break one of the rules, however
Vergne improves on his time and moves within a second of Ricciardo, as Raikkonen slots into P2 with his most recent time - six tenths off Ricciardo.
Rosberg has gone purple in his first two sectors.
And he goes fastest in the third, too. Rosberg goes fastest by over a second. That's on the soft tyre.
Hamilton has a big, big slide and manages into the run off area in Turn 5. Manages to keep it out of the barriers, though.
@LD: Well the drivers can see all that information on the dash in front of them. There's such ambiguity at the moment it's not exactly clear how it applies in fuel, to be honest.
Hamilton tells Mercedes his car briefly stalled before that slide.
Mercedes tells Hamilton: "We're investigating, Lewis." Driver tells them he had to use the clutch to get it restarted.
Hamilton sets his first timed lap and it's seven tenths down on Rosberg's earlier time. Rosberg was absolutely on it with his lap, hard to know how Hamilton's earlier moment impacted that run. Hamilton also ran wide across the final corner so he's clearly pushing like mad as well.