A player who lost in qualifying can find themselves in a main draw match on the same day. The mechanism is precise and unforgiving about timing.
The pool is defined by the loss
Lucky losers are drawn from players defeated in the final qualifying round, and eligibility usually depends on ranking within that group.
A withdrawal from the main draw after a defined point creates the vacancy, and the vacancy is filled from that pool rather than reopened to entries.
The rule exists to keep a draw full without delaying an event, since a walkover in an early round damages both the schedule and the ticket product.
Presence is a condition
Players wanting a lucky loser place must usually be present at the venue at a stated time, and absence forfeits the opportunity.
That requirement keeps a defeated qualifier at a tournament they have already lost at, sometimes for days, on the chance of a place.
It also concentrates the eligible players in one room, which is what makes a same-day draw possible.
The selection is drawn, not assigned
Where several players are eligible, a formal draw is conducted by an official, often publicly, so that the selection cannot be questioned afterwards.
Some competitions use ranking order instead, which removes the drama and the discretion at the same time.
Either way the process is documented, because the place being allocated is worth a substantial amount in prize money and ranking points.
Preparation is compressed to nothing
A player may learn of the place a short time before walking on court, having already competed and lost within the previous day.
Warmup, court allocation and equipment are all arranged in a rush, and the opponent has had a normal preparation.
Despite that disadvantage, lucky losers win often enough that the pathway is taken seriously rather than treated as a formality.
What it does to the schedule
Tournament staff hold the slot until the last permitted moment, since a returning player is preferable to a substitution.
Once the call is made, the order of play is republished and ticket holders discover the change on arrival.
Venues announce it clearly for that reason, because a spectator who bought a session for one name deserves to hear about the change from the tournament rather than from the scoreboard.

