Middlesbrough manager Aitor Karanka believes Tuesday's match against Rotherham could be their "most difficult" in the run-in.
Boro sit second in the Championship, behind Burnley, and could go top if they beat the Millers.
"I think it could be the most difficult game we have between now and the end of the season," Karanka told BBC Tees.
"It is the game where everybody thinks that we have three points in our pockets - it's a big mistake."
Rotherham sit 22nd, three points adrift of MK Dons and safety, but with back-to-back wins against Sheffield Wednesday and Brentford.
"Rotherham have won their last two games so for me now they are a different team," the 42-year old Spaniard continued.
"With their new manager [Neil Warnock], the team has character and the team is motivated now.
"They are playing at home and they are playing for their lives to stay in the Championship next season.
"So for me the biggest mistake is to believe it is going to be an easy game - for me it could be the most difficult game."
Prior to Warnock's arrival, Rotherham had lost eight of their last 12 league matches and Karanka says their turnaround in form is no accident.
Karanka added: "Sheffield are not at the top of the table but they are fighting for their play-off positions, they have a really good squad and Rotherham beat them away.
"They beat Sheffield, they beat Brentford a couple of days before - it is not a coincidence. It is because they are more motivated."