relapse, relaps

Relapse or relapse means to become ill after getting partially better; or to slip back into some former state in general--
We hope he won't relapse; he seemed to be so much better.
The speaker waited for the crowd to relapse into silence.

Relapse is also a noun ( thing or idea ), meaning the state of being ill, after seeming to recover--
He's back in the hospital; he's had a relapse.
He was doing so well; this relapse has been a shock.

Relaps or relaps ( rhymes with relapse ) is the third person singular ( he, she , it ) form of "relap", meaning to pass by on a track for the second or third ( and so on )  time--
He has lapped the favorite; if he relaps him, he's sure to win.
If she relaps him in this race, she may set a new record.

Now that you know that, you can say--
"If that runner relaps him again, he is sure to relapse into depression."