Millwall edged past League One rivals Walsall to book their place in the FA Cup fourth round where they will face Coventry at the Ricoh Arena.
There was a sparse crowd at the New Den
The home side took a 15th-minute lead after Ben May broke free to fire home and Gary Alexander produced a cool finish early in the second-half.
Walsall, who are chasing a League One play-off place, struggled to replicate their impressive league form.
Alex Nicholls did volley in for Walsall but they could not force extra-time.
Early on the game was strewn with errors but on 15 minutes the Lions took the lead.
After a melee in the middle of the field, May burst into space and fired home from just outside the penalty area despite a heavy touch from Walsall keeper Clayton Ince.
Promotion-hopefuls Walsall, missing influential skipper Tommy Mooney, looked out-of-sorts with Ishmel Demontagnac particularly wasteful.
The 19-year-old midfielder blasted wide of goal when well-placed on two occasions.
Gambian international Edrissa Sonko did go close after smuggling the ball past Lenny Pidgeley after a deep throw into the heart of Millwall's box.
With the second half just four minutes old, an unchallenged Alexander coolly curled the ball past Ince's desperate dive from an acute angle for Millwall's second.
Walsall hit back in style just after the hour when Nicholls's ambitious volley flew, via a deflection, over Pidgeley's head into the net.
As hard as Walsall battled to send the game into extra-time they could not find a goal and fell to their first defeat in 18 games.
Millwall: Pidgeley, Senda, Frampton, Robinson, Barron, Simpson (Forbes 64), Fuseini, Brammer, Gaynor (Bakayogo 72), Alexander, May.
Subs Not Used: Edwards, Callaghan, Noel.
Goals: May 15, Alexander 49.
Walsall: Ince, Weston, Gerrard, Taundry, Dann, Bradley (Boertien 68), Wrack (Dobson 65), Demontagnac (Brittain 70), Deeney, Sonko, Nicholls.
Subs Not Used: Roper, McDermott.
Booked: Demontagnac, Dann.
Goals: Nicholls 61.
Ref: Darren Deadman (Cambridgeshire).
BBC Sport Player Rater man of the match: Walsall's Scott Dann 8.40 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.