After a poor end to last year that saw them plummet down the League Two table, this was the ideal start to 2018 for the Boro, who came back emphatically after falling behind very early in the piece.
Cheltenham led inside 90 seconds when Brian Graham found Mohamed Eisa in space and the Robins’ top scorer was allowed to wander towards the area before scoring at the near post.
But Stevenage equalised halfway through the first half as Scott Flinders kept out Ronnie Henry’s deflected cross, only for Matt Godden to turn in the rebound.
The Boro then went ahead when Pett beat a hesitant Flinders to a loose ball before rolling into an empty net via the inside of the post.
The hosts had a two-goal cushion when Danny Newton slotted home after not being closed down and Pett added his second of the afternoon with two minutes left.
Pedro stepped off the bench to send Chelsea into the FA Cup semi-finals after they beat Leicester 2-1 at the King Power Stadium on Sunday evening.
RB Leipzig came from behind at the Red Bull Arena to end a run of four winless games and defeat Bundesliga leaders Bayern Munich 2-1 on Sunday.