With the rain easing and a dryer line starting to emerge with each pass of the cars on the Firestone wets, the top time had dropped over a second and a half down to 1:37.5037s even in that brief four minutes of green flag track time, with Ryan Briscoe now boasting the fastest lap so far.
Once Castroneves' Penske car was re-fired, the track went green again with around four and a half minutes remaining and Ryan Hunter-Reay immediately slashed a second off Briscoe's previous benchmark. A run-off at turn 11 for Oriel Servia was dealt with by local track workers without the need for a caution, allowing the session to make it to the chequered flag.
By that time, Hunter-Reay had shaved off a few more tenths off his laptime to set a final best lap of 1:36.0166s, almost a second better than EJ Viso, with Will Power, Justin Wilson, Rubens Barrichello and Mike Conway also getting in some decent runs by the end.
Race officials also announced that the race start would be pulled forward by half an hour, to the earliest time allowed by the television coverage window (12 noon local time, 4pm BST) in response to the likelihood of further rain later in the day.
In the UK, television coverage is via Sky Sports. Their presenter Keith Huewen confirmed on Twitter that the network would accommodate the earlier start by switching the start of race coverage onto the Sky Sports F1 channel.
Full Sunday warm-up times and positions