7 of 8. This is a great core exercise for every tennis player, as it strengthens the obliques, abs and hip flexors, and at the same time, it stretches the lower back, IT-band and chest. Lie on the ground on your back, spread your arms to the sides in a T position, pushing your palms to the ground. Lift your legs straight up toward the sky.
https://www.active.com/tennis/articles/7-core-exercises-every-tennis-player-should-do