The "Perfectly Lonely" crooner compared Swift to songwriting icon Stevie Nicks, tweeting, "Waking up to this song idea that won't leave my head. 3 days straight now. That means it's good enough to finish. t's called Half of My Heart and I want to sing it with Taylor Swift. She would make a killer Stevie Nicks in contrast to my Tom Petty of a song."

At the time, the Miss Americana star told Elle that she "freaked out" when she saw the post. "I'm really excited about just the idea that he would even mention me in his Twitter!" she said, calling herself a "big fan" of Mayer.

Their collaboration, "Half of My Heart," was featured on Mayer's Grammy-nominated fourth record, Battle Studies.

When they performed their duet at Z100's Jingle Ball in New York City, fans were quick to suspect that a romance was brewing.

Though it was months after they called it quits, the musicians had a cordial moment at the 41st annual Songwriters Hall of Fame Ceremony. Mayer presented his ex with an award, gushing, "You could put her in a time machine in any era and she would have a hit record. Don't confuse everybody loving one thing as hype. Sometimes that's everyone agreeing that it's fabulous."

When her album Speak Now was released, the lyrics on "Dear John" stood out to Swift's fans. "Dear John, I see it all, now it was wrong / Don't you think 19 is too young / To be played by your dark twisted games, when I loved you so? / I should've known," she sings.

Elsewhere in the song, she calls the John in question "an expert at sorry and keeping the lines blurry."

The "Say" artist slammed Swift's "cheap songwriting," telling Rolling Stone that he didn't appreciate being the subject of her verses. "It made me feel terrible. It was a really lousy thing to do," he said. "I never got an email. I never got a phone call. I was really caught off guard. ... [That's] abusing your talent to rub your hands together and go, 'Wait 'til he gets a load of this!' That's bulls–t."

Mayer seemingly came back with a diss track of his own in the form of "Paper Doll" from his record Paradise Valley. As listeners collected clues pointing toward Swift being the titular doll — and the ex he sings about being "22 girls in one" — he joined the conversation.

"Yeah, songwriters write songs because of people, about people," Mayer said during an interview on Today. "Anything someone else wrote is their reception of a song. I don't get involved with [it]. It's none of my business."

The Grammy winner found himself in hot water after tweeting, "Tuesday, December 13, may be the lamest day of the year, conceptually." When social media users detected shade — December 13 is Swift's birthday — he quickly cleared the air.

"NOPE. No. Nuh-uh," he wrote in a follow-up post. "People I am 39. I am deleting those tweets. Come right at me for doing so. Absolutely no shade."

Swift avoided the press during her Reputation era, but that didn't keep Mayer from weighing in on her work. He hosted a Q&A after performing at the iHeartRadio Theatre in California, telling fans that he "was thinking about reputation" when working on his song "New Light."

Mayer quipped at the time, "Not the album, but that's still fine. That's a fine piece of work."

When Swift dropped her 10th album, Midnights, she released seven bonus tracks on an extended version of the record. Fans were quick to assume "Would've, Could've, Should've" was about Mayer.

"And I damn sure would've never danced with the devil / At 19, and the God's honest truth is that the pain was heaven / And now that I'm grown, I'm scared of ghosts," she sings, before hinting, "I regret you all the time."