After Sengoku resigned, Admiral Aokiji was strongly opposed against Admiral Akainu becoming fleet admiral and fought him for the position, even though Kuzan himself originally had little interest in it.
They battled for 10 days, and their respective powers caused Punk Hazard to permanently be frozen on one half and burning on the other. In the end, Sakazuki ended up as the victor of that battle, although both men were seriously wounded.
As far as we know this is the only instance of a top tier defeating another in single combat. This would make Akainu the only character in One Piece still alive – who has actually 1v1 another top tier and emerged victorious:
Gol D. Roger vs Monkey D. Garp seemingly ended in stalemates
Gol D. Roger vs Whitebeard also ended in stalemates
Rocks D. Xebec was defeated in 2v1 against Gol D. Roger and Monkey D. Garp
Shiki was defeated in 2v1 against Garp and Sengoku
Yonko vs Yonko always ended in stalemates
Mihawk vs Shanks: no winner emerged
It’s crazy to think. But it seems possible that neither of the Yonko or Admirals(except for Akainu who didn’t accomplish this himself until he was in his fifties) have actually single-handedly defeated another top-tier in all the decades they’ve been active.