Andros Townsend finally made the breakthrough from the penalty spot on 74 minutes after Jay McEverley handled Jan Vertonghen’s cross.
The visitors from League One proved a tough nut to crack and we had 10 efforts at goal from 65 per cent of possession.
We now travel to Bramall Lane next week (Wednesday, January 28) to play for a place at Wembley.
“It was a very tough game, important to win and to go to Sheffield next week 1-0 up in the tie,” said Mauricio.
“I’m happy with the performance. It was difficult to create chances, Sheffield played deep, were very aggressive and well organised.
“It was a good test in the first game and now we realise the level Sheffield United are at and that they deserve to be in the semi-final.
“They have won games against Premier League sides Southampton and QPR and we have respect for them, but we need to go to Sheffield and try to get the victory to deserve to go to the final."