After withdrawing from a glut of tournaments due to the pandemic and a knee injury, Kyrgios has played just 24 matches over the past two years with his ranking slumping to 93rd in the world.
Philippoussis never won a grand slam but the 45-year-old still enjoyed a distinguished career, reaching the final of Wimbledon and the US Open along with winning the Davis Cup twice.
For all the ability Kyrgios has, quarter-finals at Wimbledon in 2014 and the 2015 Australian Open are the closest Australian tennis' long-time enfant terrible has got to challenging for a grand slam title.
With the first major of 2022 less than a month away, Philippoussis says Kyrgios should think carefully about how the next 12 months could affect his career.
"It's only going to get tougher for him," Philippoussis told AAP at the Jack Newton Celebrity Classic in the Hunter Valley.
"Because he plays with such power, if you don't take care of your body when you're six foot five that can lead to injuries.
"He's only 26, it's not like he's old.
"You can't be thinking you have time because sport is tough."
While talent alone has allowed Kyrgios to produce the unexpected to dig himself out of numerous holes over his career, Philippoussis fears that the has helped the Canberran has lost his on-court swagger as a consequence of his lengthy spells away from the game.
The one-time world No.13 won only seven matches in 2021, his more recent coming in July before finishing his season with five consecutive defeats.
"I don't know what's happening for him off the court," Philippoussis said.
"This year he didn't play a lot but he didn't have a good year and he lost a lot of matches.
"He lost a lot of confidence in his game this year.
"Normally you can see the confidence because he can be a break point down and he can go to his second serve.
"This year is the first time I can honestly say I saw him playing games without confidence."
Philippoussis said the solution to Kyrgios rediscovering his mojo was simple, but that the onus rested squarely on his shoulders.
"It (the chance of a slam win) is not gone," Philippoussis said.
"The beautiful thing about his game is that he can beat anyone on the day because of the weapons he has.
"He has the ability but not the consistency of the other guys. He has to grind on and off the court.
"I don't care how talented you are but you cannot rock up to tournaments without match play and training and think your talent is enough.
"He's 90 something in the world and unfortunately now he's going to have to grind to get back to the top 20."
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