Sources: Oleinik-Overeem is April 20 headliner

The UFC's return to Russia has a new main event.

Aleksei Oleinik has agreed to step in on short notice to fight Alistair Overeem in the headliner of UFC Fight Night Saint Petersburg on April 20, sources told ESPN. Oleinik is replacing fellow Russian fighter Alexander Volkov in the fight, who recently withdrew for undisclosed reasons.

Oleinik was scheduled to fight Walt Harris on May 4 in Ottawa, so the UFC will now find a new opponent for Harris.

The 41-year-old Oleinik (57-11-1) is coming off a submission win over Mark Hunt in September in Moscow. He has won two in a row.

Overeem (44-17, 1 NC) recently snapped a two-fight losing streak when he defeated Sergey Pavlovich in November.

The event will mark the promotion's first trip to Saint Petersburg and second visit to Russia.