one-on-one

one-on-one

1. Consisting of only two people, with one person's attention or energy focused solely on the other person. I make a point of offering one-on-one tutoring sessions for any student who feels like they need it. The store even has staff who give one-on-one advice on which product is right for someone.
2. In sports, (describing) a match-up in which two competitors compete directly against one another. This play is all about isolating the defender so you can go one-on-one. If they keep getting one-on-ones in the paint, they're going to dominate this game.
Farlex Dictionary of Idioms.

one on one

A direct encounter between two persons, especially a conflict, as in The two department heads went one on one regarding shelf space. This slangy expression almost certainly comes from sports. It is commonly used to refer to a two-person basketball game, but is also applied to the interaction of two players on opposing teams in football, soccer, and similar team sports. [c. 1960]
See also: on, one
The American Heritage® Dictionary of Idioms by Christine Ammer.

one on one (or one to one)

denoting or referring to a situation in which two parties come into direct contact, opposition, or correspondence.
1995 Represent I wanna speak to God one on one me and him.
See also: on, one
Farlex Partner Idioms Dictionary

one on one

A direct encounter between two persons. This expression was transferred to general use from several sports. In basketball it signifies an informal game with just two players. It also means the standard form of defense, in which one player guards one opponent. In football it similarly means a player covering (or being covered by) a single opponent. About 1960 it began to be used for nonathletic encounters, as in “Dean never liked a big party; he preferred social events where he could be one on one.”
See also: on, one
The Dictionary of Clichés by Christine Ammer
See also:
  • cooking for one
  • drive (one) out of office
  • force (one) out of office
  • force out of office
  • give (one) (one's) head
  • give head
  • give somebody their head
  • give someone their head
  • 1FTR
  • as one door closes, another (one) opens