Something's definitely not right here.
I bought a new monitor and since I started using it I can't seem to win a match anymore against player around 80% overall. Worse than that, I'm getting bageled by players against whom I always put up a fight or players I would win easily (Nalbandian, for example). This is the third loss already. Lost to Juan Monaco 6x0/6x1 (had a 2-0 record against him), lost to David Nalbandian 6x0/7x6 (had a 3x0 record against him) and now lost to Juan Ignacio Chela 6x0/6x1 (had a 1x1 record, with most recent win being mine). I couldn't possibly have become a bad player from day to night. I used to play in FullHD resolution in a monitor with max res of 1366x768. Then I started playing in FullHD in a monitor with max res 4k. I tried to play in 4k resolution to match my monitor, to no avail. Is it possible that resolution is affecting gameplay somehow? This is the only thing that changed in my setup.
EDIT: I checked my fps, it is stable at 60fps, just as before, so that's not the problem.