What if the line umpire is unsighted and the chair umpire can't decide either?

Under Appendix VII of the 2026 ITF Rules of Tennis:

1. A line umpire who can't make a call must signal this immediately to the chair umpire, who then makes the decision.
2. If the line umpire can't make the call (or there is no line umpire) and the chair umpire can't decide the question of fact either, the point is replayed.