You probably find yourself hearing the “who’s the G.O.A.T.?” debate most among NBA fans—considering LeBron James is currently staking his claim for that honor—but it’s a debate that rages across sports. Tennis fans, too, have long argued about whether we’re currently witnessing the best of all-time.

You could make a case for either of two active athletes: Roger Federer or Serena Williams. Last spring, GQ caught the internet’s wrath for declaring Federer the greatest, overlooking Serena.

For his part, Federer thinks Serena is the G.O.A.T.

“It’s been fascinating to watch,” Federer told WSJ. Magazine of Serena. “She had a totally different upbringing—I came up through Switzerland with the federation, she did it with her dad and her sister. It’s an amazing story unto itself—and then she became one of the greatest, if not the greatest tennis player of all time.”

Reporter Jason Gay asked the logical follow-up question: does he mean the greatest female player of all-time, or just the greatest overall?

“Overall,” Federer clarified.

Federer went on to add that the G.O.A.T. debates are flawed because of the different styles of play and circumstances (e.g. equipment, travel) across generations.

“It’s not fair to compare, actually,” he said.

Still, he thinks he and Serena are in elite company.

“But we know [Serena] is all the way up there,” Federer said. “I’m probably up there with somebody, somehow. Maybe there’s a group, a best of five—and if you’re in that group, you should be pleased and happy. Tennis is a funky sport when it comes to that stuff.”

Serena, 36, is third on the all-time list with 39 Grand Slam titles.