Jamie Reid scored a stoppage-time winner to send Stevenage through to the Carabao Cup third round as they beat League One Peterborough 1-0 at the Lamex.
Both sides came into the tie in fine form, the home team unbeaten all season and the visitors with five wins from six.
League Two Stevenage had shocked Championship's Reading 2-1 to reach this stage, while the Posh won at Plymouth.
Stevenage edged the first-half, although each side had a golden chance to take the lead.
Carl Piergianni flashed a clearance straight to Frankie Kent, who hit the side netting with half the goal to aim at.
Then a similar opportunity presented itself to Stevenage debutant David Amoo, but the former Liverpool youth player blazed the ball over on the stretch.
Just minutes after Jonson Clarke-Harris was brought on, fellow substitute Jack Marriott squared the ball for him to finish before Peterborough's top scorer was ruled offside.
But Stevenage nearly hit straight back, Saxon Earley denied at the far post by a flying Will Blackmore.
And in the 82nd minute, Aaron Chapman produced a sublime double save to stop Marriott.
But Reid grabbed the winner in the third minute of added time, sweeping home Luther Wilden's cross to vanquish Grant McCann's men.
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