Correct me if I'm wrong but I think the only Admiral that is confirmed to ever be defeated was Aokiji, and it took another Admiral to do it.
It's a stark contrast from defeating a Yonkou. Yonkous are known worldwide for losing all the time, Kaidou who is dubbed as the strongest has an all time untouchable losing record.
It'll be a pretty big deal if an Admiral ever lost to a pirate. Zoro and Sanji if they ever managed to accomplish such a feat would be in extremely rare company.