HD 165052 is a short-period massive eccentric binary system that undergoes apsidal motion. As the rate of apsidal motion is directly related to the internal structure constants of the binary components, its study allows getting insight into the internal structure of the stars. We use medium- and high-resolution spectroscopic observations of HD 165052 to provide constraints on the fundamental properties of the binary system and the evolutionary state of its components. We apply a spectral disentangling code to reconstruct artefact-free spectra of the individual stars and derive the radial velocities (RVs) at the times of the observations. We perform the first analysis of the disentangled spectra with the non-local thermodynamic equilibrium model atmosphere code CMFGEN to determine the stellar properties. We derive the first self-consistent orbital solution of all existing RV data, including those reported in the literature, accounting for apsidal motion. We build, for the very first time, dedicated stellar evolution tracks with the Cles code requesting the theoretical effective temperatures and luminosities to match those obtained from our spectroscopic analysis. The binary system HD 165052, consisting of an O6.5 V((f)) primary (T-eff,T-P = 37 500 +/- 1000 K) and an O7 V((f)) secondary (T-eff,T-S = 36 000 +/- 1000 K), displays apsidal motion at a rate of (11.30(-0.49)(+0.64))degrees yr(-1). Evolutionary masses are compared to minimum dynamical masses to constrain the orbital inclination. Evolutionary masses M-ev,M-P = 24.8 +/- 1.0 M-circle dot and M-ev,M-S = 20.9 +/- 1.0 M-circle dot and radii R-ev,R-P = 7.0(-0.4)(+0.5) R-circle dot and R-ev,R-S = 6.2(-0.3)(+0.4) R-circle dot are derived, and the inclination is constrained to 22.1 degrees <= i <= 23.3 degrees. Theoretical apsidal motion rates, derived assuming an age of 2.0 +/- 0.5 Myr for the binary, are in agreement with the observational determination. The agreement with theoretical apsidal motion rates enforces the inferred values of the evolutionary stellar masses and radii.