The temperature in Boca Raton had soared above 90 degrees, but on a side court at the Evert Tennis Academy, Naomi Osaka was just digging into one of her last training sessions before the summer hardcourt season. Wearing leggings and a tank top — her magnificent mane of frizzy blond-tinted hair emerging from the back of her Adidas cap — the 20-year-old smacked crisp topspin groundstrokes with her c