The Spanish world number two, playing his first match after a month lay-off, eventually won his Group B opener 6-2 3-6 7-6 but he needed to rush off court early in the deciding set suffering with what he called a "terrible stomach.”
The 10-times grand slam champion was having enough trouble subduing tournament debutant Fish as it was before feeling ill.
However, despite losing three games in a row after returning from the toilet, he displayed his trademark fighting spirit to avoid a defeat that would have left his chances of reaching the semi-finals in real jeopardy.
Afterwards he explained what had happened.
"I really need to come back to hotel and rest a little bit because I played for one hour suffering a lot," he told reporters. "Because when you don't feel well, you have to keep running, all the muscles are more tired than usual.
"I don't know what it was. I just ate some pasta before the match. For the lunch, I eat some pasta with salmon at the hotel. So nothing strange and nothing unusual.
"But during the match, I start to feel something half of the second set. But really getting worse and worse at beginning of the third. I asked to the umpire that I needed to go to the toilet. I say to him, I need to go to the toilet. Can I go at the next changeover? "He told me I had to go before my serve. So that's why I had to go with the 2-0." Nadal said he hoped to be in better shape by the time he plays great rival Roger Federer on Tuesday.
"I am worried about getting better for the practice of tomorrow. I think two days are enough," he said.
Nadal took advantage of a slow start by his opponent to dominate the first set but the 29-year-old Fish responded by winning the second on his fifth set point with one of many sumptuous volleys he produced in the near three-hour duel.
Nadal moved 2-0 ahead in the decider but after he rushed off court for a toilet break Fish reeled off the next three games when he returned.
The Spaniard broke back and had two match points at 5-6 on the Fish serve, failing to convert either chance.
Perspiring heavily on the quick indoor surface, Nadal moved 4-1 ahead in the tiebreak and when three more match points arrived he made no mistake, sealing victory when the leaping Fish netted a high volley.
"I felt very bad from the stomach since the beginning of the third," Nadal said. "I was seriously really lucky for the victory, and I for sure a.m. very happy."
