Central to the understanding of high-temperature superconductivity is the evolution of the electronic structure as doping alters the density of charge carriers in the CuO2 planes. Superconductivity emerges along the path from a normal metal on the overdoped side to an antiferromagnetic insulator on the underdoped side. This path also exhibits a severe disruption of the overdoped normal metal's Fermi surface1, 2, 3. Angle-resolved photoemission spectroscopy (ARPES) on the surfaces of easily cleaved materials such as Bi2Sr2CaCu2O8+ shows that in zero magnetic field the Fermi surface breaks up into disconnected arcs4, 5, 6. However, in high magnetic field, quantum oscillations7 at low temperatures in YBa2Cu3O6.5 indicate the existence of small Fermi surface pockets8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18. Reconciling these two phenomena through ARPES studies of YBa2Cu3O7- (YBCO) has been hampered by the surface sensitivity of the technique19, 20, 21. Here, we show that this difficulty stems from the polarity and resulting self-doping of the YBCO surface. Through in situ deposition of potassium atoms on cleaved YBCO, we can continuously control the surface doping and follow the evolution of the Fermi surface from the overdoped to the underdoped regime. The present approach opens the door to systematic studies of high-temperature superconductors, such as creating new electron-doped superconductors from insulating parent compounds.