Feliciano Lopez and Bernard Tomic were eliminated in the Ecuador Open quarter-finals on Friday.
Lopez, the Spanish second seed and last year's runner-up, was trailing countryman Albert Ramos-Vinolas 7-5 4-3 when he retired having called for the trainer numerous times.
Ramos-Vinolas progressed to the last four, with the official reasoning for Lopez's withdrawal listed as fatigue.
Awaiting Ramos-Vinolas in the semis of the clay-court tournament in Quito is defending champion Victor Estrella Burgos.
Estrella Burgos overcame Argentine qualifier Renzo Olivo 6-4 6-7 (5-7) 6-4 in two hours, 15 minutes.
Top seed Tomic was also sent packing as he went down to Italian Paolo Lorenzi 6-7 (5-7) 7-5 6-3.
The sixth-seeded Lorenzi served 25 aces and was not broken by the Australian.
Lorenzi will face Thomaz Bellucci after the Brazilian third seed battled past Pablo Carreno Busta 7-6 (7-0) 4-6 6-2.