WBO cruiserweight champion Oleksandr Usyk beat WBC title holder Mairis Briedis by a majority points decision to unify the belts.
The two met in the semi-final of the World Boxing Super Series tournament and Usyk won 115-113, 115-113 and 114-114 in Latvia.
He will now meet the winner of IBF champion Murat Gassiev against WBA belt holder Yunier Dorticos in the final.
The event will conclude in May and features a $50m (£35m) prize fund.
That pool will be split across two tournaments, one for super-middleweights and one for cruiserweights, with each featuring eight fighters competing from a quarter-final stage.
Whoever wins the cruiserweight section will hold all four belts.
Usyk's hard-fought win over Briedis extended the 31-year-old's career record to 14 fights without defeat.