Defending champion Naomi Osaka made a flying start to the U.S. Open as she downed Czech Marie Bouzkova 6-4, 6-1 at a packed Arthur Ashe Stadium on Monday.

In the men's tournament, Greek third seed Stefanos Tsitsipas struggled past Andy Murray in five sets, while Daniil Medvedev defeated Frenchman Richard Gasquet.

Osaka, seeking her third U.S. Open crown in four years, stretched her Grand Slam winning streak to 16 matches as she brushed past 87th-ranked Bouzkova to book a second-round matchup with Serbian qualifier Olga Danilovic.

"It feels kind of crazy to play in front of everyone again," Osaka said. "I feel really comfortable here. I'm just glad I won."

Osaka beat Bouzkova in their only prior meeting in the first round of this year's Australian Open on the way to her fourth Grand Slam title.

The 23-year-old Japanese star could become the first back-to-back U.S. Open women's champion since Serena Williams, out this year with a torn hamstring, won her third in a row in 2014.

Osaka's major win streak includes her most recent U.S. and Australian Open wins and a first-round win at this year's French Open before withdrawing over mental health issues.

Osaka, who lit the cauldron at the Tokyo Olympics, gave an Olympic pin to a young girl in the same venue where she won last year's title when fans were banned due to COVID-19.

"It felt quite lonely for me," she said. "So I'm quite glad to see little kids in the audience and grownups too. The energy here is unmatched."

Osaka broke Bouzkova in the 10th game of the first set when the Czech netted a backhand, the broke for a 2-0 lead in the second set on the way to victory in 93 minutes.

Tsitsipas sees off Murray

Tsitsipas outlasted Britain's Murray, a three-time Grand Slam champion, by 2-6, 7-6 (9/7), 3-6, 6-3, 6-4 after 4 hours and 49 minutes in surroundings more like an intense second-week showdown than a curtain-raiser for the fortnight.

"To have an electric atmosphere out here is something we've been waiting for," Tsitsipas said.

Two-time Olympic champion Murray ranked 112th lost for the first time in 15 first-round U.S. Open matches.

"It didn't come easy," Tsitsipas said.

Tsitsipas, this year's French Open runner-up, took a long break before the final set, frustrating Murray, who surrendered a break in the opening game. Delay tactics brought a frosty reception at the net after the match.

"I have zero time for that stuff at all and I lost respect for him," Murray said. "It's nonsense. And he knows it, as well."

Tsitsipas, who plays Frenchman Adrian Mannarino next, said he followed ATP rules on breaks and medical timeouts, even as Murray questioned the length and timing.

"I'm playing by the rules and sticking to what the ATP says is fair," said Tsitsipas. "Then the rest is fine."

In the second-set tiebreaker, Murray slipped at the net and fell because his sweat-soaked shoes were wet and he didn't have another pair. Murray missed out on two set points and the 23-year-old Greek star won five of the next six points to grab the set and level the match.

"The shoes got so wet that at the end of the set, I was slipping basically and was losing balance," Murray said. "That was my bad. It was an important moment in the match."

Murray broke in the second game of the third set and held out from there, but Tsitsipas took the last two sets to advance.

Crowd energy returns

Spectators had to show proof of vaccination to attend but they brought the energy back to Ashe.

"Playing without fans here was brutal," 2017 U.S. Open winner Sloane Stephens said. "Having these fans out and the energy, the atmosphere, it brings a lot back to tennis."

Two-time Grand Slam champion Simona Halep and Stephens each won to reach the second round.

"You feel the energy. You feel alive on the court," Halep said. "Hopefully it will stay like that forever."

Russian second seed Daniil Medvedev defeated Frenchman Richard Gasquet 6-4, 6-3, 6-1 for his 200th career match win and 160th win on hardcourts.

Women's second seed Aryna Sabalenka of Belarus outlasted Serbia's Nina Stojanovic 6-4, 6-7 (4/7), 6-0 while Russian men's fifth seed Andrey Rublev ousted 221st-ranked Ivo Karlovic 6-3, 7-6 (7/3), 6-3.