Novak Djokovic battled a stomach bug in a narrow 4-6, 7-6 (2), 6-4 win on Thursday which put the top seed into the quarter-finals of the Paris Masters.
The 36-year-old said he was feeling far from his best in a defeat of Tallon Griekspoor which took more than two and a half hours.
Djokovic was hoping that he will recover enough to put in a battle on Friday in his last-eight match.
“If he had won it would be deserved,” the Serb said., “I started out well but ran out of steam.
“I’ve been struggling the last few days with my stomach. I played a good tiebreak in the second set (after losing the first).
“In the third I started feeling better. I hope to be fit to fight the next challenge.
“But I can’t predict how I will feel tomorrow. I hope I have the energy to battle for three sets.
“The event is only gonna get tougher from here.”
Djokovic reached the final a year ago, losing to Holger Rune.
The 24-time Grand Slam champion who produced 28 winners to 43 for his opponent, advanced on the first of three match points in a roller-coaster final set.
Griekspoor strung together five straight games in the opening set, winning it from a 1-4 start but was unable to halt the Djokovic juggernaut.
Djokovic, bidding for a seventh indoor title here, lost a 4-2 lead in the third set, but broke to love for 5-4 before serving out the win.
