Richard Marx Discovered an Old Song He Wrote with Keith Urban, and Decided to Finally Let It See the Light of Day

Richard Marx has known Keith Urban for decades, but the pop music extraordinaire still has a hard time totally understanding what's going on in the mind of the country music superstar.

"When we write a song, Keith is always over the moon about it on Monday and by Wednesday he's not into it anymore," Marx, 58, admits with a laugh during a recent interview with PEOPLE. "It goes from, 'This is the greatest song that's ever been written by any two people ever' to 'I have no interest in this song whatsoever.' So I'm used to the sort of bipolar thing that happens when I write with him."

And while the fellow Grammy winners have seen much success together as writing partners for Urban's hit songs such as "Everybody," "Better Life," and the No. 1 smash "Long Hot Summer," there was one particular song that Urban never seemed to warm up to.

Rick Diamond/Getty Richard Marx and Keith Urban

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"We wrote it at his house starting at 10 a.m. and by 12:30 p.m., the song was almost done," Urban says of the inspiring song "One Day Longer," premiering exclusively on PEOPLE. "I remember Keith saying to me, 'Ricardo, I love this song but I'm really hungry right now and I hate everything when I'm hungry.' A couple days later, he was no longer into it."

For years, the song simply took up space in a folder on Marx's laptop, until the accomplished singer/songwriter went back and gave it a listen one more time. And now, it's this song that serves as the first of five country songs from Marx's forthcoming album Songwriter, set for release on Sept. 30.

"I sent Keith a voice note saying that I was putting out 'One Day Longer,' and he texts me back and he says, 'I have no recollection of that song, but it sounds really good,'" Marx laughs. "He sounded like he couldn't believe that we would write something so good together."

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The two creative visionaries first started working together when Marx was just dipping his creative toe into Nashville's thriving songwriting community. And during one of those visits, Marx met Matt Rollings, who produced Urban's first album in the states back in 1999.

"Matt would always say how he was working with Keith and that he reminded him of me, even though he was country," says Marx, laughing at the memory. "He thought we would hit it off."

And the two ended up doing just that. The next time Marx was in Music City, he met Urban for an afternoon writing session that ended up lasting two days. However, there wasn't much songwriting going on.

"We never finished a song," laughs Marx, who at the time was hitting the big time as a solo artist with songs such as "Hold on to the Nights" and "Don't Mean Nothing." "We just ended up shooting pool and talking at his house. But yeah, that's when this amazingly successful story began."

As with everything else Marx does, he did get the thumbs up on "One Day Longer" from wife Daisy Fuentes.

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"Daisy loves this one," says Marx of his wife, who has also lived her life in the spotlight as an actress, model and business owner. "And I would say that that's not a given with her. She's honest when it comes to the music that I do."

A multi-genre project that has Marx delving into everything from country to rock to pop, Songwriter not only serves as yet another creative outlet for him, but an excuse to write with some of the finest out there, including Burt Bacharach, Darius Rucker and Chris Daughtry.

"The chameleon part of me as a songwriter has always been there and it's always been natural," explains Marx, who also released his best-selling memoir Stories to Tell last summer. "The fact that the fans seem to be loving the new music is why I keep writing songs. That's the muse. Not that I ever needed a lesson in gratitude, but it has actually risen, just by getting to watch the fans react to this music."