Bristol City, who are managed by Nigel Pearson, have been unbeaten in all competitions since the last week of December, moving up to 13th in the second-tier Championship.
The last time the two teams faced each other at Ashton Gate, Manchester City edged Bristol City 3-2 in the League Cup semi-finals second leg in 2018 en route to Guardiola's first trophy with the English club.
"From my experience in this country, in the last six years, when you go (play) away in the FA cup at a Championship team, it is always, always tough. (Playing at) home is a little different," Guardiola told reporters ahead of Tuesday's clash.
"Last 12 games - no defeat. That means they are there. My scouts are saying it's a good team. The manager has experience, did really well with Leicester (City) and Watford when he was there one or two seasons ago.
"It's a final. The FA cup is a really, really important competition, so it's a serious team we will play tomorrow."
Manchester City bounced back from two consecutive draws to beat Bournemouth 4-1 over the weekend but Guardiola feels the team can play better as they sit two points behind league leaders Arsenal, who have a game in hand.
"The way we are playing is good, results could be better. Sometimes it happens in football," Guardiola said.
"I have no complaints about how we are playing and how we are fighting. The mood in the locker room is exceptional and the guys are doing everything - that is enough for me.
"I have the feeling in the Premier League many things are going to happen, everyone is going to drop points. But we cannot drop any, we want to fight until the end."
Guardiola confirmed influential midfielder Kevin De Bruyne is still suffering from "a little bit of illness" after sitting out the trip to Bournemouth while he will also take a late call on defenders John Stones and Aymeric Laporte.
