This shot made by Pablo Carreño Busta (Spain, age 30) in his five-set match against Tallon Griekspoor (Netherlands, 25) at the Australian Open is perfectly legal but very rarely seen.
Busta won the match after 4 hrs 10 mins of play.
You are not allowed to touch the net, or to jump over the net to play a ball, to return a shot. You can run around the net post and play from there, provided your feet/ your racquet does not touch the inside of the opponent’s singles court.
It is OK to reach over the net to hit a shot, while not touching it, nor touching the inside of the opponent’s singles court lines.
Busta, quick as he is, saw what was happening, followed the ball by running AROUND the net post, got his racquet to the ball, and made it bounce inside the singles court on Griekspoor’s side. (Griekspoor had no hope of getting to it. The ball went into the net on Griekspoor’s side after it had bounced once.)