Apollo approached William Hill to discuss a deal for the $3 billion market-capitalization company, and discussions are still at an early stage, sources told Bloomberg.
Caesars has also approached William Hill about a takeover, according to Bloomberg, and both parties have made separate cash proposals for the company.
Five weeks ago Jefferies analyst James Wheatcroft, in a note reported by Barron's, wrote that William Hill shares were inexpensive because investors weren't taking account of the U.K. company's U.S. sports-betting franchise.
At last check William Hill American depositary receipts were 35% higher at $15.29.
Apollo already has a history in gambling investments following a leveraged buyout of Harrah's Entertainment in 2008. Harrah's was subsequently renamed Caesars Entertainment.
Last year, Apollo acquired a stake in Italy's Gamenet Group SpA.
Earlier this month, it was reported that William Hill is looking to merge its U.S. business with Caesars Entertainment, which has long been a partner for William Hill.
William Hill U.S. Chief Executive Joe Asher told Bloomberg the company is in discussions with Caesars about combining their sports-betting and online-gaming businesses.
"There’s a lot of opportunity in there, and we think that we’ve got some really powerful assets in this space, so obviously it’s an ongoing subject of discussion,” Asher said in an interview.
Caesars has said that the U.S. sports and online-gaming operations of the two combined could generate $700 million in revenue next year.
Caesars already owned 20% of William Hill’s U.S. arm under in a deal signed two years ago with Eldorado Resorts, which merged with Caesars in July.
Caesars' takeover of Eldorado extended that deal to iconic properties like Caesars Palace. About 15 additional locations should be added in coming weeks, including the Horseshoe in Council Bluffs, Iowa, and the Harrah’s in Atlantic City.
They would bring William Hill’s total to 170 retail locations in 13 states, the company said.