Martin Plowman was a surprise pace-setter in the second IndyCar practice session at Infineon Raceway on Saturday.
The Sam Schmidt Motorsport driver was one of several to capitalise on the extra grip from the softer-compound red slick tyres on offer to move up the order, and the Brit finished the session with a best lap of 1m19.0350s.
Friday pace setters Dario Franchitti and Will Power were just behind him for the Ganassi and Penske teams respectively, both title contenders having opted to stay on the harder rubber.
Behind them, JR Hildebrand (Panther Racing) and James Jakes (Dale Coyne Racing) used the reds to go fourth and fifth quickest respectively.
A late yellow flag triggered by a spinning Ana Beatriz potentially derailed a few drivers' hopes of improving their times.
The track returned to green with a couple of minutes remaining, but there was not enough time for any significant changes to the order.