NEW YORK: On a muggy night, Rafael Nadal said he felt dehydrated, and that's why his level of play dropped for about a set.

“I had some problems. I was sweating a lot,” Nadal said after beating 18-year-old Borna Coric in four sets in the US Open's first round.

Nadal said he “had a little bit of (a) stomach problem” and wasn't “physically perfect,” which is why he lost the third set, before feeling stronger in the fourth.

“I recovered (a) little bit. I played, again, more aggressive,” Nadal added.

He said he doesn't think it was a big deal and shouldn't affect him Wednesday in the second round against Diego Schwartzman.

Turnaround for Nadal?

Rafael Nadal managed to win his first US Open match since 2013, despite dropping a set along the way.

Nadal, who collected two of his 14 Grand Slam titles at Flushing Meadows, moved into the second round with a 6-3, 6-2, 4-6, 6-4 victory over 18-year-old Borna Coric of Croatia in Arthur Ashe Stadium.

The eighth-seeded Nadal missed last year's US Open because of a right wrist injury. He won the tournament in 2010 and 2013.

Coric, ranked 33rd, is the youngest member of the ATP's top 50. He had won his only previous matchup with Nadal, which came last year.

Serena spies on Federer

Successful as she is in her own right, even Serena Williams relishes a chance to get an up-close look at Roger Federer on a tennis court.

When they practice near each other, Williams - owner of 21 Grand Slam singles titles - takes a peek at what Federer - owner of 17 - is doing.

“I'm always looking over: 'What is he doing?' Giving him the side eye. 'OK, maybe I should do that. I should do that. Oh, he's taking a break now,'” Williams said after moving into the second round of the U.S. Open on Monday night.

“It's super distracting,” Williams added.

“I totally look at him. I mean, he's Roger.”

Federer plays his first-round match at Flushing Meadows on Tuesday.

Monfils feels the pain

A back injury forced 16th-seeded Gael Monfils to stop playing his first-round match against Illya Marchenko while trailing 2-6, 6-4, 5-0, 30-0.

Monfils fell onto his back early in the second set.

“I couldn't play, couldn't move, couldn't do anything,” said Monfils, twice a quarterfinalist at the US Open and a crowd favorite at Flushing Meadows.

He has bothered by his back throughout the summer. He lost in the second round at Montreal and the first round at Cincinnati earlier this month.

27-minute win

Serena Williams began her bid to complete a calendar-year Grand Slam by moving into the second round of the US Open when her opponent, 86th-ranked Vitalia Diatchenko of Russia, stopped playing because of an injury with the American ahead 6-0, 2-0.

The match lasted only 27 minutes. Williams won 32 of the 37 points they played.

Diatchenko was visited by a trainer during the first set and got her left foot and ankle taped.

Next for Williams is a match against Kiki Bertens of the Netherlands.

Williams is trying to become the first player to win all four Grand Slam tournaments in a season since Steffi Graf in 1988.

The stunning mid-game interview

CoCo Vandeweghe beat fellow American Sloane Stephens in straight sets in the first round of the U.S. Open - and stopped to talk about it in the middle.

The 45th-ranked Vandeweghe won 6-4, 6-3 to follow up her breakthrough run to the Wimbledon quarterfinals, where she pushed Maria Sharapova to three sets. After winning the first set Monday, Vandeweghe agreed to do an interview with ESPN, an unusual move that had other players chirping on Twitter.

“Did I just see Coco do an interview on court, mid match, after the first set??” Caroline Wozniacki tweeted.

“Surely you would wanna focus on the game out there? No?”

Vandeweghe kept her focus and said in an on-court interview after that match: “In general, I'm a sports fan. For anybody to have an insight into an athlete's mind is a positive.”

For the 29th-seeded Stephens, it was her third first-round exit in her past six Grand Slam tournaments, after she reached at least the fourth round in six straight before that.

Vandeweghe next faces another American, 101st-ranked Bethanie Mattek-Sands. The winner of their match could meet Serena Williams in the third round.

Djokovic hardly breaks a sweat

Top-ranked Novak Djokovic needed just 71 minutes to win his first-round match at the US Open.

Djokovic dropped only three games in beating 91st-ranked Joao Souza of Brazil 6-1, 6-1, 6-1 on Monday. He didn't face a single break point.

Cilic overcomes Pella

Marin Cilic started the defense of his US Open title with a straight-set victory.

Cilic beat 94th-ranked qualifier Guido Pella 6-3, 7-6 (3), 7-6 (3) on Monday, looking comfortable with his status as the reigning champ. Cilic was the surprise winner last year for his first major title.

He hasn't quite been able to find that same form since, hindered by injuries, and is seeded ninth. He was the first reigning US Open men's champ since 2001 to not begin his title defense at Arthur Ashe Stadium, playing at Louis Armstrong instead.

In the previous match on the court, the man Cilic beat in last year's final, fourth-seeded Kei Nishikori, lost, opening up Cilic's quarter of the bracket for another potential long run.

Venus finds it hard

Venus Williams won a tough three-set match on a hot day, bouncing back after failing to close out the win in the second.

The 35-year-old Williams, the oldest woman in the field, has never lost in the first round of the US Open. The seven-time major champ beat 85th-ranked Monica Puig of Puerto Rico 6-4, 6-7 (7), 6-3 on Monday to improve to 17-0 in her openers in New York.

Williams had a chance to serve out the second set, then wasted four match points in the tiebreaker.

Undaunted, she promptly broke Puig's serve to open the third and stayed ahead from there to win in 2 hours, 40 minutes.

The 21-year-old Puig reached the fourth round at Wimbledon in 2013, but has now lost in the first round at her past three major tournaments.

Top women contenders crash out

The first day of the US Open isn't over, and Serena Williams' potentially toughest semifinal opponents are already almost all gone.

The quarter of the draw that was supposed to feature third-ranked Maria Sharapova is running out of seeded players fast. A day after Sharapova withdrew because of a lingering right leg injury, four of the seven remaining seeds in the quarter lost early Monday: No. 7 Ana Ivanovic, No. 10 Carla Suarez Navarro, No. 21 Jelena Jankovic and No. 30 Svetlana Kuznetsova.

Only No. 13 Ekaterina Makarova and No. 17 Elina Svitolina had won, with No. 25 Eugenie Bouchard yet to play.

Nishikori nightmare

A year after reaching his first Grand Slam final at the US Open, Japan's Kei Nishikori was eliminated a few hours into the tournament.

The fourth-seeded Nishikori lost in five sets to 41st-ranked Benoit Paire on Monday. The Frenchman saved two match points in the fourth-set tiebreaker and went on to a 6-4, 3-6, 4-6, 7-6 (6), 6-4 victory.

Nishikori withdrew from the hard-court warmup at Cincinnati because of a hip injury but said Saturday he was fine physically. On Monday, though, he looked sluggish late in the match.

Paire had never before defeated an opponent ranked in the top five.

Last year at Flushing Meadows, Nishikori became the first man from Asia to reach a Grand Slam final, where he lost to Marin Cilic.

No retirement for Fish

Mardy Fish won't be retiring just yet.

The former top-10 player will stick around at the US Open for at least a couple of more days after winning his first-round match at what he says will be his final tournament.

Fish got a favorable draw against 102nd-ranked Marco Cecchinato of Italy, who was making his Grand Slam debut and is now 0-7 in tour-level matches.

Fish, whose has struggled with anxiety disorder, decided to wrap up his career at the US Open to try to make some new memories at his home Grand Slam tournament.

In 2012, Fish had to withdraw before his fourth-round match against Roger Federer because of a panic attack. He hadn't played at Flushing Meadows since.

“I'm glad I got to come back here one more time,” the 33-year-old Fish said in an on-court interview after his 6-7 (5), 6-3, 6-1, 6-3 win Monday.

The American showed some nerves trying to serve out the first set, but otherwise Cecchinato looked overmatched. Fish requested to play on the Grandstand, the tournament's third-largest court, where the fans can sit close and get loud.

He was cramping at the end of the nearly 3-hour match and said afterward it had probably been a couple of years since he had been on court that long. But he closed it out in style with an ace before Cecchinato could make things interesting.