Osaka had come into Friday with a 16-match winning streak at majors and four career trophies, all on hard courts.
Another possible factor in her failure to close things out while serving for the victory against the left-hander Fernandez at 6-5 in the second set: Osaka hadn’t competed since Monday.
She grabbed 12 of the last 13 points, including the last nine, with a break at love to go up 6-5, and a hold at love with the help of a pair of aces at 112 mph and 114 mph to end it.
Osaka seemed on track for a similar conclusion in the second set, leading 6-5 and serving.
She fell behind 5-0 in the ensuing tiebreaker, missing shots and displaying her frustration, as she occasionally has done in the past, by flinging her racket.
Fernandez’s knee-to-the-ground, quick-redirect style at the baseline is reminiscent of another lefty, Angelique Kerber, a three-time Grand Slam champion who won the 2016 US Open.