Go, go, become #1
(I'll do my best to become #2)
Actually FanDuel does not have great odds on Tennis, the juice is too high. But the futures are definitely beatable. You should be killing the ATP 250 and WTA 250 tournaments.
DraftKings has terrible odds on tennis (as in, you shouldn't even bother to look) but they have a ton of promos. It's all about earning freebets and using them on juicy longshots. The longer the better (well, as long as they're juicy. Most 50-1 longshots return maybe 20 cents on the dollar). If you can find double digit longshots that return 80 cents or more on the dollar then they're good enough to use freebets on.