The Tigers have been the big success story of Super League to date having opened up a six-point lead at the top of the table by playing an exhilarating brand of attacking rugby.
Belief the club can land a first major trophy since 1986 is certainly growing but Powell has sounded a note of caution before this weekend's tough trip to the KCOM Stadium.
Castleford's last defeat came at that venue in April while the Black and Whites will not want to relinquish the cup they won in glorious fashion last year.
Powell said: "This is a different entity - it is a one-off game, a knockout. If you get beaten, that's it, you're done in the Challenge Cup.
"We don't want that to happen but we recognise Hull will be desperate as well. They are the holders, they are at home and you'd think they'd be slight favourites.
"I'm mindful of us needing to have great discipline about us and put them under pressure. That's something we didn't do last time we played them."
Cas had a double near-miss in 2014 when they were beaten at Wembley by Leeds and fell just short of the League Leaders' Shield after losing their final regular season game.
Yet this year they look an even tougher proposition with their free-flowing attack not only delivering - top Super League tryscorer Greg Eden already has 30 to his name after claiming five last week - but with their defence remaining disciplined too. Their points difference of 345 after 18 games is more than 200 better than any other side.
Powell said: "We are in a good place. Since Hull beat us we have won eight straight and there is a lot of confidence about the players.
"And I don't see any dropping off and patting themselves on the back. It looks like a group hungry to do well across the season."
Hull have been less consistent but, fourth in the table, they have been one of the season's stronger sides.
Cup fever also remains high after the club finally ended their Wembley hoodoo by winning there at the ninth attempt in a pulsating final against Warrington last August. More than 12,000 tickets have been sold for what will be Hull's highest cup attendance for seven years.
Key players Danny Houghton, Mark Minichiello, Gareth Ellis and Marc Sneyd could also return.
Coach Lee Radford said: "They've obviously been the most consistent team over the season but we are a good side on our day and we're expected to be at our best for Sunday.
"We don't need prods to get up for this game. We didn't go through what we went through last season to give it up without giving a good account of ourselves."
Fast Results
Scores
Racecards 