Reigning US Open champion Naomi Osaka confronted severe nerves in a rocky start to her title defence Tuesday while men’s contenders Dominic Thiem and Stefanos Tsitsipas tumbled out in the first round.

Osaka suffered a first-round exit last month at Wimbledon and the Japanese star struggled to put away Russia’s Anna Blinkova in her opener at Arthur Ashe Stadium before prevailing 6-4, 6-7 (5/7), 6-2.

The top seed goes on to face Poland’s Magda Linette, who captured her maiden WTA title in the Bronx last week.

Osaka must retain her Flushing Meadows crown to have a chance of remaining world number one and was tested extensively by 2015 Wimbledon junior finalist Blinkova, who raced 4-1 ahead in the first set and saved a match point in the second to force a decider.

Wimbledon champion Simona Halep, ousted in the opening round the past two years at Flushing Meadows, outlasted US lucky loser Nicole Gibbs 6-3, 3-6, 6-2 while two-time runner-up Caroline Wozniacki rebounded from a woeful start to beat Wang Yafan in three sets.

Two-time French Open runner-up Dominic Thiem was upset by Italian giant-killer Thomas Fabbiano 6-4, 3-6, 6-3, 6-2 as three top-10 players were eliminated from Rafael Nadal’s half of the draw.

Greek eighth seed Tsitsipas struggled with cramps and accused umpires of having “preferences” after he slumped to a 6-4, 6-7 (5/7), 7-6 (9/7), 7-5 defeat by Next Gen rival Andrey Rublev in a gruelling four-hour slog.

Like Thiem, it was a second successive Grand Slam first-round exit for Tsitsipas, who was hit with a point penalty in the final set for a time violation.

“This chair umpire, I don’t know, he has something against me. I don’t know why,” said Tsitsipas, who snapped “you’re all weirdos” at French official Damien Dumosois during his match. (AFP)