The second-ranked Nadal began feeling pain in his abdomen late Saturday, and the next day went to the hospital where he learned he had the early stages of appendicitis.
Nadal said on Tuesday he chose to try treatment with antibiotics rather than undergo surgery right away. He felt well enough to begin hitting again, he said.
The 14-time major winner said the risk of his condition getting worse after three days of steady improvement "is always very, very low."
"I am not going to go for surgery. That's great news for me today. That's why I'm here practicing for 45 minutes, and trying to play tomorrow. But obviously I am not in my best condition," he said.
Nadal said he will talk with his doctors when he returns to Spain after the tournament about whether he will need to have the appendix removed.
"My idea is take it out, but you can imagine that I didn't (think) much about that yet. I am here in Shanghai. I just want to try. Then when I go back home, I will talk about the situation with my doctor," he said.
The Spaniard is playing just his second tournament since returning from a two-month layoff due to a wrist injury. After missing the summer hard-court season, including the defence of his title at the U.S. Open, Nadal said it was important to get back on the court as soon as possible so he's back in top form before the Australian Open in January. He reached the China Open quarterfinals last week.
"I don't expect anything from these two tournaments. I expect just to come here and play again and practice and feel a little bit again the competition, try to get in rhythm as soon as possible," he said.
Nadal plays his opening match on Wednesday against countryman Feliciano Lopez.
In first-round play, Vancouver's Vaesk Pospisil advanced with a 6-2, 6-7 (5), 6-1 win over Colombia's Santiago Giraldo. Pospisil had 18 aces in the match and broke Giraldo six times.
Pospisil plays Spain's Roberto Bautista Agut in the second round.
Meanwhile, Fabio Fognini of Italy lost to a Chinese wild card making his ATP Tour debut, then bumped into him at the net and gave an obscene gesture to the crowd.
The No. 17-ranked Fognini, a 10-year professional, was upset by the 553rd-ranked Wang Chuhan 7-6 (5), 6-4.
After the two shook hands, Fognini gave Wang a little shove with his arm as they crossed paths, then flashed his middle finger to the crowd upon exiting the stadium.
The fiery Italian, who was fined $27,500 at Wimbledon for outbursts during a first-round match, has lost five of six matches since reaching the Cincinnati quarterfinals in August.
Wang said he knew Fognini's game somewhat after practicing with him in Beijing last week, but didn't have a lot of confidence heading into the match.
"When I was practicing with him before, I found he's more fast-paced than me," he said. "This match has really, most importantly, helped me get closer to this kind of feeling of competing with top-100 players. Hopefully, next time when I meet a top-100 player, I won't feel so nervous or feel that it's impossible to win."
He next plays Tunisian qualifier Malek Jaziri, ranked 87th, for a spot in the third round.
Two-time major winner Andy Murray was broken twice in the second set but recovered to defeat Russian qualifier Teymuraz Gabashvili 6-1, 7-5.
Murray is playing for the third straight week in hopes of qualifying for next month's ATP finals in London. After winning a title in Shenzhen two weeks ago and reaching the semifinals of the China Open, he now sits in ninth place in the standings, just outside the eight-man field.
"In these three weeks, mentally, it can be challenging to concentrate match after match after match," Murray said. "Each day when you get up, you have to be on it, you have to be ready and prepared."
Other winners included Mikhail Youzhny of Russia, Gilles Simon of France and Ivan Dodig of Croatia.