Federer is a master of the Forehand, in this exhaustive video, his stance of choice is semi-open 90% of the time, almost all of the high balls between his abdominal and shoulder are hit semi-open. He hits closed stance on the forward lateral run, open stance on the run, and even a couple of forehands extreme open stance approach shots on the stretch. The majority of short balls around the service line he hits square(neutral) stance or closed. Hits a few square stance FHs from the baseline. Every single FH he hits is SITUATIONAL, he is heavily persuaded by a personal preference or style to go semi-open. His rotation is timed and synced perfectly with the rhythm and speed of the incoming ball. Just a side note, EVERY SINGLE one-handed topspin backhand is closed and most of those backhands set up his forehand winners. His backhand stance on the slice/underspin changes based on time pressure and what he’s trying to achieve with respect to holding on to good court positioning.
I teach every stance, and every teachable moment includes as a foundation the “situational” or “tactical” approach to take time away from your opponent, maintain good court positioning, and/or height/depth of the incoming ball. If you are quick enough with the feet and anticipation skills are dialed in, any stance can be effective. Most of my juniors and adults are not playing rally balls at speeds in excess of 90mph, so I’ve witnessed many players that simply come to me with sloppy and inefficient footwork, they are lazy and play with an extreme open stance instead a rotating a bit and hit a more solid shot semi-open or neutral stance to discipline themselves to maintain a solid finishing position. Finishing in a “BALANCED” position is crucial to setting up for the NEXT ball. Again, if I keep in mind the player, their tendencies, speed DNA, and overall athleticism, our training together will ultimately lead to a player “playing” the ball instead of just becoming a “hitter” of the ball.