The 73-year-old singer, who is a coach on the BBC1 show, whisked semi-finalists Sally Barker and Bizzi Dixon down to Covent Garden market in London to perform for passers-by.
Barker brought her own acoustic guitar and she and her team-mate - and rival - Dixon sang to a large crowd which gathered to watch.
And to ensure he did not let down the fans who stopped to watch, Sir Tom belted out his hits It's Not Unusual and Kiss.
Spectators were chanting "Team Tom, Team Tom" as they watched the performances.
A programme insider said: "Sir Tom brought Covent Garden to a standstill - it was incredible. He really showed Bizzi and Sally how to do it.
"The impromptu busk-off was a great success and helped Sir Tom get the public behind his semi-finalists."