Novak Djokovic has admitted to form and fitness concerns heading into Sunday’s start of the French Open where the title holder could lose the world number one spot to Jannik Sinner. Djokovic, who turned 37 earlier in the week, is yet to win a tournament this year and went out in the Geneva semi-finals against Czech player Tomas Machac, 6-4, 0-6, 6-1. The Serb spoke of stomach problems but also agreed that he is not going into the Paris grand slam, where his first opponent is French wildcard entry Pierre-Hugues Herbert, in the best overall shape. “Of course I am worried. I haven’t been playing …