Tennis is arguably the worst value sport to be a professional in the Top 10 (by money metrics).
That's why despite me being able to coach my son for free up to a decent level (which would save thousands) I concentrated on basketball (which I played in college so I can help too).
Tennis' problem is a monumental wealth gap where the peak earnings are comparable to top athletes around the globe but there's a massive drop-off.
I was familiar with Peter Polansky (high #110) & Frank Dancevic (high #65), Canadian pros who lived in my area and I was able to meet routinely because of clubs, etc. Now I didn't know their finances and they weren't exactly struggling but with the amount of work they put in they weren't rolling in the dough so to speak. At least not directly from their tour earnings. Officially you have 1.8 and 1.4 mill in career prize money but you might as well cut that in half from the onset in terms of money in pocket. Travel expenses and tournament fees as well as staff which you need at least 3 of (medical, trainer, coach) are not covered and over the course of a calendar year let alone a career you're going to be bleeding a lot of money, not to mention taxation.
Sponsorships help subsidize costs but they often require attendance at events and usually more taxable than prize money from tournaments. Now in Canada you could use staff costs as tax deductibles so at the end of the day you might limit your overall income losses but there will still be losses.
Biggest ways to minimize loss to tax is residency. Now here comes the reality. The bigger earners are going to be the ones who can afford to live in places such as Monaco or the Bahamas. And those are the guys who can afford to do so with their sponsors. A Canadian pro is less likely to get away with sponsorships living away from Canada since they'll be more reliant on local endorsements. Dudi Sela, an Israeli tennis player has done well for himself concentrating on Israel, and it being a small country, transportation isn't much of an issue.
Some will say being in the Top 100 means good money but that's not at all a rule as players have even lost money despite having a Top 100 year, and how consistent is that ranking you got to ask. But more to the point, how does it make sense to spend untold amounts of money for development, equipment and travel to MAYBE be one of a hundred players to earn good income playing the sport?
In basketball, you can be the 2,000th best player and earn good money. Hell top players OUTSIDE the NBA will earn multi-millions in salary (pre-tax of course). Then you hear of stories where in Algeria for some fourth tier league a good player by their standards will be earning $8,000 USD a month, which let's face it is pretty good money to play maybe 7 or 8 games in a country where your money will go farther. And I guess it depends how you look at it. In Argentina they play 38 regular season games over the course of 6 months. I've only known of a few Canadians who played down there but the salaries for the year were around $65,000USD and included free condo paid by the team, you also got fed at the facility most of the time. Taxation is around 35% so I don't know the details of money in pocket but even if it was $30,000USD that's $40,000cdn for 6 months of work. I mean, that's pretty damn good for a pro athlete way down the totem pole.
By comparison, unless you're really smart with your scheduling, have no staff and have good concentration of winning you're simply not making it long on the ATP without a sponsor. And that's the end of it really, tennis is very much a rich sport where talent alone doesn't cut it. To be frank the sport loses a TON of talented players to other, more sensible money sports.