Nigel Clough has warned the Blades face a 'challenge' when Notts County provide the opposition at Bramall Lane on Friday night.
The Magpies, under the stewardship of Shaun Derry and his assistant Greg Abbott, are just one place behind United in the League One table and head to South Yorkshire unbeaten on the road.
Clough also believes the two best goalkeepers in the division will go head-to-head on the night.
Looking ahead to the clash, the Blades boss said: "Notts County have had a brilliant start and we aren't surprised, we know Shaun and Greg quite well and they have picked up from where they left off last season.
"As a player you could always see that Shaun would probably go on to be a manager and he will have learnt a lot from the likes of Neil Warnock too over the years.
"On Friday night you will see the two best goalkeepers in the division on opposing teams, Mark Howard and in particular Roy Carroll for Notts County have been instrumental to the success we have enjoyed respectively this season.
"There are only two teams out of 92 that are unbeaten away from home, one is Chelsea, the other is Notts County, that tells you all you need to know about the challenge ahead of us."