The Blades had earler led before going 2-1 and 3-2 behind before John Egan's 94th-minute equaliser forced a replay at Bramall Lane.
United’s Daniel Jebbison was sent off as the Blades finished with 10 men, and here’s how we rated the United side in Wales...
13. Subs: Max Lowe 5
Gave United more attacking impetus down the left side after coming on Photo: Gary Oakley
14. Billy Sharp 5
Looked so angry with himself after the ball bounced off him and fell perfectly for Wrexham to score but led the line well in search of an equaliser Photo: Gary Oakley
15. Iliman Ndiaye 5
Came on for Osborn and almost lit up the game with a superb dribble before his shot was deflected over by a teammate Photo: Gary Oakley
16. Ismaila Coulibaly n/a
Came off the bench for another short cameo appearance Photo: Gary Oakley