Cape Town - Former seven-time
Impey became just the second South African to win a stage of the Tour de France when he sprinted to victory in Sunday's 170.5km Stage 9 between Saint-Etienne and Brioude.
Robbie Hunter is the only other South African to have tasted success in cycling's greatest race when he won Stage 11 of the 2007 race.
Impey, riding for Australian outfit Mitchelton-Scott, was part of a 15-rider breakaway which ended up contesting stage honours after the peloton deemed them no threat to overall yellow jersey contention.
Impey, 34, drew praise from Armstrong, who previously raced alongside him at Team RadioShack.
Speaking on his podcast, , Armstrong said: