Photo: Anne-Christine Poujoulat/AFP
The 32-year-old Wawrinka, the winner of three Grand Slam titles, was 6-4, 1-1 down to the qualifier when he retired.
Swiss star Wawrinka, now down at 13 in the rankings, missed the last six months of the 2017 season after undergoing knee surgery.
He has endured a traumatic start to 2018, losing in the second round of the Australian Open to unheralded Tennys Sandgren of the United States.
After making the semifinals in Sofia, he then slumped to an embarrassing defeat to Dutch world number 259 Tallon Griekspoor in Rotterdam last week.