Roger Federer described Novak Djokovic's record-breaking 23rd Grand Slam title achievement as "unbelievable" on Wednesday but avoided anointing his old rival as the greatest player of all time.

"I think what Novak has done is unbelievable," said Federer, who racked up 20 Grand Slams before he retired last year.

Djokovic edged ahead of 22-time major winner Rafael Nadal when he won his third French Open in Paris earlier this month.

Federer, 41, speaking at the Halle grass-court tournament on Wednesday, admitted however it was "hard to say" who was best player of all time.

"The whole matter is difficult to answer," said Federer. "I asked a friend, what is more difficult, winning Wimbledon at 17, like Boris Becker, or the French Open at 36 like Novak?"

"I don't know. It's a great time to be a tennis fan but also a player," added the Swiss great who was guest of honour in Halle for 'Roger Federer Day'.

"It means a lot to me," Federer said of his return to Halle, where he won a record 10 singles titles.

"Of course I'm overjoyed. I was able to shape this tournament myself with so many victories. It feels like a home away from home."

Federer spent over an hour signing autographs and posing for selfies on centre court in the sun but said he did not miss being in the heat of the action.

"Of course, you want to experience that again. But as long as you know your body cannot do it at this level, you don't feel the need to be out on the court," he said.