Judenapier,
want to echo the serious warning from David.
Please don't be so addicted to tennis that you fail to care for yourself properly. Please be very very grateful for a successful fusion without postoperative problems .
Have 2 neighbours who also thought they could play again and both ended up causing complications, further pain and additional spine surgeries.
Just saying, it's not worth it, in my humble opinion, get another hobby, but definitely not golf which also causes spine problems