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.