Bari have won just one of their past 20 fixtures and lie at the foot of the standings, 12 points behind safety with 11 games remaining.
In-form Udinese will be overwhelming favourites on Sunday, but Guidolin insists they can take nothing for granted.
"I expect a lively Bari and a difficult game for us," Guidolin said. "I don't think about the past results but about the present and I believe we will be put to the test by a side that cannot afford a defeat.
"In football, there is nothing guaranteed."
Udinese head into the encounter sky high after hammering Palermo 7-0 in Sicily in the last round of matches.
The result extended Udinese's unbeaten run to 10 games and left them fifth in the standings, just one point behind fourth-placed Lazio, who hold the last Champions League qualifying spot.
"We are going through a great spell," Guidolin said. "But we have to wait and see if it will continue.
"The game at Palermo was worth three points and we have to put that result behind us and focus on taking another win on Sunday.
"The important thing is for my players to maintain the focus as we are not used to being in the limelight."
Guidolin is expected to field the same line-up that thrashed Palermo, with Antonio Di Natale and Alexis Sanchez playing in attack.
The two players combined to score the seven goals at Palermo.
Bari coach Bortolo Mutti welcomes back defender Marco Rossi, who missed last weekend's 1-1 draw against Fiorentina through suspension.