Naomi Osaka made a shaky start to her US Open title defence as she edged out Anna Blinkova with an unconvincing 6-4 6-7 (5-7) 6-2 victory at Flushing Meadows.
Osaka rose to prominence at last year's US Open, dropping just one set throughout the tournament before overcoming Serena Williams in a final overshadowed by the latter's dispute with chair umpire Carlos Ramos.
The world number one, who crashed out in the first round at Wimbledon, made 50 unforced errors on her return to Arthur Ashe Stadium, though, and - bar a five-game winning streak in the first set - struggled to exert any control over Bronx Open quarter-finalist Blinkova.
Osaka became the first defending women's champion to be taken to a third set, but the 21-year-old's mistakes did not come back to haunt her thanks to Blinkova's failure to convert three break points in the third set.
That good fortune handed Osaka the impetus she needed, and the two-time grand slam champion booked a second-round clash with Magda Linette when she converted a second match point to finally break Blinkova's resolve.
Blinkova raced into a three-game lead - Osaka, wearing a knee brace following an injury which forced her out of the Western & Southern Open - failing to find any rhythm and struggling to land her first serves.
But Osaka hit her stride in game six, going on to win 11 points out of 12 to draw level at 4-4.
After a 14-shot rally went the top seed's way, Blinkova bounced back with a sublime winner, only to lose the game with an overhit backhand, and a venomous cross-court forehand handed the defending champion the set.
Osaka's troubles with her first serve continued, though, and she had to claw back break points in successive service games.
The momentum seemed to turn in Osaka's favour when she fought back from three set-points down before breaking serve, only for Blinkova to rally herself and force the tie-break, which the Russian clinched when her opponent clipped into the net.
Blinkova's exasperation became evident as she failed to capitalise on two break points in the third game of the final set, and the world number 84's frustrations were compounded when she lost her next service game.
Having finally forged ahead, Osaka saved another break point to go 4-1 up, and although Blinkova held serve with a fantastic volley, it was not enough to halt her opponent, who settled an exhilarating, two-hour, 28-minute contest in her favour with a thumping down-the-line forehand.
Naomi Osaka  bt Anna Blinkova 6-4 6-7 (5-7) 6-2.
Osaka – 44/50
Blinkova – 15/22
Osaka – 8/1
Blinkova – 1/4
BREAK POINTS WON
Osaka – 5/8
Blinkova – 2/11
FIRST SERVE PERCENTAGE
Osaka – 57
Blinkova – 64
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Osaka – 68/52
Blinkova – 58/60
Osaka – 110
Blinkova – 102