Kei Nishikori withdrew on Tuesday from the ATP tournament in Estoril, depriving the 250 event of a drawcard.
The Japanese fourth seed cited an abdominal muscle strain he picked up during training in the coastal centre near Lisbon.
The withdrawal of Asia’s top player boosted Roberto Carballes Baena into the second round through Nishikori’s bye, with the Spaniard set to face Kevin Anderson.
Nishikori was to have been making his tournament debut.
“I got to town early, took a few days off and had two great training sessions,” a statement read.
“However, yesterday I stepped badly and had a pain in my right adductor muscle.
“We are doing check-ups and I don’t think it’s anything serious, but I won’t be able to play my game within 48 hours. I hope to be able to return to the tournament soon, ”
Nishikori remains uncertain for the Madrid Masters starting on Monday. The 2014 US Open finalist did not play for a year due to a series of injuries which began in New York in late August, 2019.