Skiing is one of those black-hole sports that spine integrity cannot be predicted. You could ski all day on groomed slopes stopping before muscle fatigue and be just fine. However, if conditions worsen and you catch an edge, you fatigue prematurely, get hit by another individual or take an unexpected fall, the abnormal loading forces could injure the disc further.
Tennis is better but still carries risk due to the BLT component of the ground-stroke.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.