Most tournaments run a separate competition before the main draw begins. Understanding it explains a good deal about early-round results and about how careers are built.
Direct entry is decided by ranking
Main draws hold a fixed number of places, and most are allocated to the highest-ranked players who enter by the deadline.
A handful are reserved for wildcards, awarded by the tournament to players returning from injury, local competitors or names that sell tickets.
The remaining places go to qualifiers, and that small allocation is what an entire preliminary tournament competes for.
The qualifying field is deeper than it looks
Because places are scarce, players ranked well inside the professional tour often find themselves in qualifying at the largest events.
The standard is therefore high, and a qualifier reaching the main draw has usually beaten opponents comparable to those in the early rounds ahead.
This is why qualifiers cause upsets more often than their ranking suggests, and why seeded players treat the fixture seriously.
Match load cuts both ways
A qualifier arrives having already played several matches while his first opponent has played none, which means he is match-sharp and adapted to the conditions.
He is also carrying accumulated fatigue into a tournament that may require several more matches, with less recovery time than everyone around him.
The advantage tends to show in the first round and the disadvantage later, which is the pattern the results generally follow.
Lucky losers add a further route
When a player withdraws after the main draw is set, a place is filled by a player who lost in the final qualifying round.
Selection among eligible losers is made by ranking or by draw depending on the rules in use, and it happens with very little notice.
Players in that position must remain at the venue and ready to compete, which is why qualifying losers often stay on site rather than travelling on.
Why the structure matters for careers
Ranking points and prize money at the largest events dwarf what is available at smaller ones, so a single successful qualifying week can fund months of travel.
The reverse is also true: losing in the final qualifying round produces a fraction of the reward for nearly the same amount of tennis.
That cliff edge is one of the sharpest financial features of the sport, and it sits entirely outside what most spectators ever see.