Nick Kyrgios says Bernard Tomic has "lost his way" and that it's unfair to lump him in the same category as Australia's fallen tennis star.

As he prepares to lead Australia into Davis Cup battle with Belgium in Brussels, Kyrgios has opened up on his relationship with Tomic, who remains in exile with former teammates after a disastrous 2017 campaign.

The two former junior grand slam champions were jostling for Australia's top ranking last year before Tomic tumbled to 146th in the world and Kyrgios soared into the top 15 for the first time this season.

While the enigmatic Kyrgios admits he's still "not the professional tennis needs me to be", the 22-year-old insists Tomic is an even greater conundrum.

"You'd also be wrong if you tried to lump me in the same category as Bernard Tomic, as [Rio Olympics Australian chef de mission] Kitty Chiller and tons of others have over time," Kyrgios told playersvoice.com.au.

"Bernie has lost his way. We were pretty good mates when I was younger.

"I obviously didn't know the tennis tour too well back then and we were guys of similar age, representing the same country, on the road at many of the same tournaments.

"But a lot has changed since then. He needs to figure out what he wants to do. I can't relate to anything he says anymore.

"He says one thing and he does the other. And he contradicts himself all the time.

"He says tennis doesn't make him happy, that he doesn't really like the game, yet he says the only thing that will really make him happy is winning a grand slam. It doesn't make sense at all."

Kyrgios, who was banned from the ATP Tour last year for not giving his best efforts in a lame loss at the Shanghai Masters, concedes he's not perfect, either.

But he's trying to get it together under the the Davis Cup leadership of trusted mentor Lleyton Hewitt.

"When I'm in the right frame of mind, I feel unbeatable," he said.

"That period this year where I beat Novak Djokovic a couple of times in a row in Acapulco and Indian Wells. Yep. Unbeatable.

"It's easy to get up for a match like like that - big-name opponent, centre court, huge challenge. I love that.

"It's against the lower-ranked guys on the back courts where I can't get it together and tank.

"Obviously, my grand slam season has been terrible. I meant what I said straight after losing to John Millman in New York.

"Maybe it's time for [coach] Sebastian Grosjean to work with someone more dedicated than me. I don't know."

AAP