San Jose, California. The 18-year-old, who reached her first Grand Slam final at the French Open in June, needed just one break of serve to seize the first set 6-4 in the blockbuster encounter.
With booming serves and matching ground strokes, she then roared to a 5-1 lead in the second set. But the Japanese superstar, unseeded in her first tournament since an opening-round exit at Roland Garros, didn't go down without a fight.
She saved match points -- all on her own serve, before Gauff was finally able to serve it out. Down 5-1 and 0-40, Osaka uncorked a string of big serves that kept Gauff at bay.
Gauff would get one more chance in that game but hit a backhand service return into the net before Osaka fired her first ace of the match for a game point on the way to a hold of serve.