 Complete the following phrase:  Third drivers are still used occasionally, most notably Sebastian Vettel drove for the BMW Sauber team on Fridays at the start of the 2007 season, Scottish driver Paul di Resta ran in eight Friday practice sessions for Force India during the 2010 season. Also, on several occasions, third drivers have acted as reserves, racing if one of the teams two race drivers is injured or otherwise unable to race, as in di Restas case during the 