In late 2008, Sato took part in tests at Jerez with Scuderia Toro Rosso, to become a candidate to fill the seat vacated by Sebastian Vettel. He was competing against former Toro Rosso driver Sébastien Bourdais and Red Bull Racing test and reserve driver Sébastien Buemi for one of the two race seats. He first drove on 18 September, more than four months since Super Aguri's withdrawal, and tested for the team again for two days in November, setting the fastest time on the 17th, 3 tenths ahead of Buemi, and proceeded by setting the second-fastest time on the 18th. The race seat was eventually given to Bourdais, and in March 2009 it was announced that Sato would not be the reserve driver for the Red Bull team.