Tennis legend Pete Sampras has lauded Novak Djokovic and concedes he did not anticipate the Serbian's dominance of the men's game.
Djokovic's French Open triumph earlier this month completed a career Grand Slam - something Sampras never managed, with the Roland Garros crown the one missing from his collection.
Djokovic now holds all four major crowns, and heads into the start of his Wimbledon campaign on Monday looking for a third straight success at the All England Club.
And Sampras, who won 14 grand slam titles, admits he could not have envisaged the extent of Djokovic's domination.
"Mentally he is a bull and it's just been amazing, I didn't see it coming," he told Omnisport.
"I always thought Novak was a little up and down emotionally and lose his way but now he is just boom, back into it and it's fun to watch. He doesn't have quite the same presence as Roger [Federer] or Rafa [Nadal] but as a competitor, I just admire what he's been able to do and be so consistent.
"He's at the top of his game, he's figured it all out. This two, three-year run has just been some of the best tennis in the history of the game.
"What he has been able to do against Roger, beating him four times, two at Wimbledon and two at the [US] Open. He's done it all. Some of the greatest tennis of all time.
"You feel as though he may slip up or lose his confidence for a little bit but week in week out he's always at the end. He's very talented, the best mover in the world, he fights hard. He's got great strokes, he has the whole package."