The start of a quarterback battle is never more than a practice play in the Lions’ training camp, and a fresh one has sprung up with less warning than a summer thunderstorm.

A week ago, figuring out the depth chart was as easy at 1, 2, 3 — Jon Kitna, Dan Orlovsky and rookie Drew Stanton, in that order.

After four days of camp, the order is now written in pencil, with a large eraser handy.

Kitna is secure as the starter, but the backup job is up for grabs with a newcomer making a push.

J.T. O’Sullivan, signed by the Lions earlier this month, has jumbled the depth chart with an unexpectedly strong showing. O’Sullivan and Orlovsky suddenly are in a hot battle to be Kitna’s backup.

• O’Sullivan becomes factor in Lions’ QB race