Eight-time Grand Slam winner Agassi endured a four-year barren spell between winning the 1995 Australian Open and 1999 French Open, during which his ranking fell to 141st in the world.
The American wrote about his battle to regain his form in his autobiography in 2009. Briton Raducanu, 23, has not reached the last eight of a major since her shock 2021 US Open triumph.
"I would encourage her to focus on being the best of herself on a daily basis, but don't judge that by results or thinking that somehow you have to be 100%," Agassi told the BBC on Wednesday.
"You have to learn to be 100% of whatever percent you're at (and) be comfortable in your own identity inside of that, not what people say."
Japan's Naomi Osaka, who won her fourth Grand Slam in 2021, also took inspiration from Agassi's book last year, when she reached the US Open semi-finals for the first time in five years.