"1989, you should've won. You should've got the decision," Robinson told Hearns, as the boxing greats stood face-to-face on an L.A. sidewalk Monday, talking to TMZ Sports.

We talked to 65-year-old Leonard and 62-year-old Hearns as they were leaving Doug Ellin and Kevin Dillon's "Victory The Podcast" ... where they commemorated the 40th anniversary of their first fight in 1981 (Ray won by 14th round TKO ... handing The Hitman his first L).

But, it wasn't the 1st fight we were discussing ... it was the highly disputed rematch that followed 8 years later in 1989.

The fight was very close ... and the scorecards proved as much. One judge scored the fight 113-112, Robinson. Another scored it 113-112, Hearns. Meanwhile, a third judge had it all knotted up at 112-112 ... meaning the fight was declared a draw.

No official winner ... despite the fact man fans and the television announcers believed Hearns won fair and square.

And, now, decades later, Sugar Ray himself is admitting he lost the rematch.

We asked Tommy if it was the first time Ray admitted he should've won ... and sure enough, Hearns said it was.

"Yes, the first time. It's not gonna be the last time," The Hitman laughed.

Bottom line ... these Hall of Famers are two of the greatest fighters who have ever lived, and it's awesome to see they're buddies all these years later!