The singer surprised fans in Zürich, Switzerland as she played the breakup anthem inspired by that brief relationship -- from her album "Speak Now" -- on piano Tuesday night ... and mashed it up with "Sad Beautiful Tragic."

Taylor parted ways with Joe back in 2008 ... so it's highly unlikely he was on her mind all these years later. A better reason for her to play it is the fact the song's lyrics mention Tuesday's date, July 9th.

The failed romance is probably not a memory Taylor wants to dwell on too much. After all, she hinted back then he broke up with her over the phone in just 25 seconds. Ouch!

But, years have flown by and it looks like it's all water under the bridge -- Taylor even struck up a friendship with Sophie Turner while she was happily married to Joe ... later extending a NYC crash pad to her when their divorce news hit.

But, these days, it's all about Travis for Taylor. Her BF's pulling out all the stops ... showing up to many of her shows and proving he's her number one fan.