I think Barnett handles him, but there's so many questions with him you'll never know if he won't get replaced by an alt.
In that case, Sergei should beat any alternate or Bum Rogers and definitely make the finals.
I wouldn't call Rogers a bum. He was finishing everybody until he bumped into Ubereem and Fedor.... it's not like those two are cans...
The only alternate I see giving Sergi fits is Del Rosario.
I don't see Barnett getting past him tbh.