Global university rankings influence students' choices and higher education policies throughout the world. When rankers not only evaluate universities but also provide them with consulting, analytics, or advertising services, rankers are vulnerable to conflicts of interest that may potentially distort their rankings. The paper assesses the impact of contracting with rankers on university ranking outcomes using a difference-in-difference research design. The study matches data on the positions of 28 Russian universities in QS World University Rankings between 2016 and 2021 with information on contracts these universities had for services from QS--the company that produces these rankings. The study compares the fluctuations in QS rankings with data obtained from the Times Higher Education rankings and data recorded by national statistics. The results suggest that the universities with frequent QS-related contracts had an increase of 0.75 standard deviations (~ 140 positions) in QS World University Rankings and an increase of 0.9 standard deviations in reported QS faculty-student ratio scores over 5 years, regardless of changes in the institutional characteristics. The observed distortions could be explained by university rankers' self-serving bias that benefits both rankers and prestige-seeking universities and reinforces the persistence of rankings in higher education.