The venue is nearly finalized for the Andy Ruiz vs. Anthony Joshua rematch.
Last month, Ruiz pulled off a shocker when he dropped Joshua four times to claim the IBF, IBO, WBA, WBO heavyweight titles at Madison Square Garden in New York City.
It wasn't long before Joshua exercised his contractual clause for an immediate rematch.
The second fight is being targeted for a date on December - with Cardiff and New York City and the two locations being considered.
But Eddie Hearn, who promotes Joshua, says Cardiff is still the favorite to host the event - but the O2 Arena in London is also a potential option for the fight.
Ruiz would the fight to take place in New York City or Mexico - but Joshua is the one who holds control of where the second bout takes place.
The exact location should be known in the next few days.
"This week we will be letting Team Ruiz know the venue," Hearn told Sky Sports News. "Whether you hear it or not, well someone at Sky will hear it, but whether the wider media hear it or not, I'm not too sure how that announcement will work.
"Right now the front-runner is still Cardiff, a marginal front-runner, but seeing Saturday night at The O2 and the atmosphere, and seeing the response to this, we've had a record pre-sale at The O2 for Lomachenko-Campbell, I believe this fight will sell out today.
"But seeing the support of the British public, I kind of start thinking to myself that Joshua-Ruiz at Cardiff in front of 80,000 people would be something very, very special."