  Direct dynamical mass measurements of stars with masses above 30 M${}_\odot$
are rare. This is the result of the low yield of the upper initial mass
function and the limited number of such systems in eclipsing binaries.
Long-period, double-lined spectroscopic binaries that are also resolved
astrometrically offer an alternative for obtaining absolute masses of stellar
objects. 9 Sgr is one such long-period, high-mass binary. Unfortunately, a
large amount of tension exists between its total dynamical mass inferred from
radial velocity measurements and that from astrometric data. We obtained the
astrometric orbit from VLTI/PIONIER and VLTI/GRAVITY interferometric
measurements. Using archival and new spectroscopy, we performed a grid-based
spectral disentangling search to constrain the semi-amplitudes of the radial
velocity curves. We computed atmospheric parameters and surface abundances by
adjusting \textsc{fastwind} atmosphere models and we compared our results with
evolutionary tracks computed with the Bonn Evolutionary Code (BEC). Grid
spectral disentangling of 9 Sgr supports the presence of a 53 M${}_\odot$
primary and a 39 M${}_\odot$ secondary. Comparison with BEC evolutionary tracks
shows the components of 9 Sgr are most likely coeval with an age of roughly 1
Myr. Our analysis clears up the contradiction between mass and orbital
inclination estimates reported in previous studies. We detect the presence of
significant CNO-processed material at the surface of the primary, suggesting
enhanced internal mixing compared to currently implemented in the BEC models.
The present measurements provide a high-quality high-mass anchor to validate
stellar evolution models and to test the efficiency of internal mixing
processes.
