Reigning U.S. Open champion Naomi Osaka confronted severe nerves in a rocky start to her title defense 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.

"I have a lot of really good memories here because I grew up in this area. I don't think I've ever been this nervous in my life," said Osaka, sporting a black brace on the left knee that prompted her to retire in Cincinnati.

"You want to do well after you did well last year. I definitely didn't want to lose in the third set."

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.

"I knew it was going to be a really tough battle. The only thing I would have wished is that I won the first match point," Osaka said.

Nadal in powerful start

Second-seeded Rafael Nadal looked dominant in his first-round U.S. Open match, easily defeating Australian John Millman 6-3, 6-2, 6-2.

Nadal, who is undefeated in U.S. Open first rounds, may have had reason to worry about Millman, who upset Roger Federer last year in the fourth round at Flushing Meadows. But the 60th-ranked Australian could not compete with Nadal from the baseline. The Spaniard capped his workmanlike victory with a clean forehand winner.

Next up for Nadal is another Australian, Thanasi Kokkinakis, a wild-card entry ranked 203rd.

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.

"I got very tired and exhausted after two sets. I'm far away from 100 percent," Thiem said. "It was not the real me there on the court."

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 grueling four-hour slog.

Gauff shakes off nerves

Wimbledon teen sensation Coco Gauff shook off nerves and rallied Tuesday to reach the second round of the U.S. Open, defeating Russia's Anastasia Potapova 3-6, 6-2, 6-4.

The 15-year-old American, who ousted idol Venus Williams on her way to the fourth round at Wimbledon, drew energy from a cheering crowd at Louis Armstrong Stadium to advance.

"It was because of the crowd," Gauff said. "No matter where I was on the court, I could always hear somebody supporting me and I'm grateful for that."

It was an atmosphere that forced Gauff to soak in the moment and accept the nervousness that followed.

"It was crazy," she said. "I was nervous going out on the court, such a big court, and my home Slam, I wanted to do well. It was a great atmosphere to play in and a great experience for me."

Gauff faces Hungarian qualifier Timea Babos on Thursday with a possible showdown against top-ranked defending champion Naomi Osaka of Japan in the third round.

"I first met Naomi (when) I hit with her at the Miami Open maybe three years ago," Gauff said. "Our dads always knew each other. When I talked to her, we do have similarities. She's doing amazing, obviously. Hopefully I can get to her level. I mean, she's amazing."

(With input from AFP, AP)