Millers boss Ronnie Moore admitted his Division One side had a lucky escape after earning a replay following a 1-1 draw against Northampton at the Sixfields.
Rotherham looked on course for a third 1-0 FA Cup success against the Cobblers in the last 11 years when they went in front 10 minutes after the re-start, with Richard Barker slotting home a Chris Sedgwick free-kick.
But the Cobblers had other ideas and equalised eight minutes later when Martin Smith rapped home his eighth goal of the season after Mike Pollitt had spilled a hanging cross by Chris Hargreaves.
The Cobblers had a glorious chance to seal the tie 13 minutes from time when lively striker Derek Asamoah sprinted clear, but, after lobbing the ball over Pollitt, he fired into the side netting from 15 yards with the goal at his mercy.
Moore said: "Someone up there was smiling on us today. We were hanging on at the end. At least we have a second chance to put things right." His opposite number Colin Calderwood was full of praise for Asamoah who also had two penalty appeals turned down, although he was booked for the second of them for an apparent theatrical approach.
Calderwood said: "Derek is very naive and enthusiastic and I know it is not in his nature to go looking for penalties like that.
"As for the chance he missed, he did a lot of things right to get into that position. We have to accept he isn't going to score them all. It was a pity because that would have been a huge goal for us and what the player and the team deserved today." Both goalkeepers had to make telling saves, with Pollitt keeping out a 23rd minute drive from Hargreaves, while Lee Harper also kept the scoreline blank 10 minutes before the break when he punched Sedgewick's curling free-kick out of the top corner.