Gervonta "Tank" Davis was paying close attention when champ Teofimo Lopez lost his belts and undefeated record last weekend ... telling TMZ Sports he learned a valuable lesson, don't underestimate your opponent!
"Just a mistake he made," Davis says, "Going in there thinking someone's a slouch."
Tank added ... "That just taught me to never go in there thinking I won already and I didn't."
Of course, 24-year-old Lopez, the former unified lightweight champ, lost to George Kambosos Jr. last weekend ... in what was a major upset.