According to the San Diego Union Tribune, several people reported that Nelson had barely made it into the song when he started coughing and appeared to be having trouble breathing.
In response to a Monday inquiry from the Union-Tribune, Nelson's longtime publicist said the iconic singer-songwriter had either "a bad cold or the flu", and was planning to recuperate at his home in Texas.
Fans who purchased tickets to the aborted Harrah's show will receive refunds.
An email explaining the cancellations from Ticketmaster says in part that "u$3 nfortunately, due to illness, Willie Nelson's performance concluded early".
The 84-year-old returned to his ranch in Texas to recover, calling off two dates at the Cosmopolitan in Las Vegas, as well as a concert in Laughlin, Nevada.
The representative also confirmed that Nelson would be canceling the remainder of his shows scheduled for January. The Stay All Night (Stay a Little Longer) is next primed to hit the stage for a February 7 concert at the Macon City Auditorium in Macon, Georgia.
"This is Willie", he tweeted after that health problem. "I am sorry to have to cut the SLC show short tonight The altitude got to me I am feeling better now & headed for lower ground".