The positive covid-19 test came after returning to Spain.
By then, Rafael Nadal had tested himself in both Abu Dhabi and Kuwait and received negative answers as late as last Friday.
But now he feels sick.
"I feel uncomfortable sometimes but I hope I feel better," he writes.
"I stay at home and have communicated the results to everyone I have had contact with."
The matches in Abu Dhabi were Nadal's first in four months, after recovering from a long-term foot injury.
In the first match, the 35-year-old was defeated by the British Andy Murray and in the second, against Denis Shapovalov, there was also a loss.
Despite this, Nadal was happy to be back on track.
- It was a positive start.
"I played better than I expected a few weeks ago," he said of the first match of his return, according to tennis.com.
It has been uncertain whether the Spaniard, who has 20 grand slam titles, would have time to be restored to the Australian Open, which begins on January 17 in Melbourne.
But as the injury situation eased, Nadal had planned to play both the major tournament and the team competition ATP Cup in Melbourne, which starts two weeks before.
Now he has to wait with his plans, he writes on Twitter.
"The situation has resulted in me being forced to have total flexibility in my calendar, and I will analyze my options based on how my mind develops".