NEW YORK – Spinning in 70 mph second serves, grabbing at his hamstring during points, Andy Murray of Britain gritted his way through head-to-toe cramps to win at the U.S. Open.

Murray outlasted Robin Haase of the Netherlands 6-3, 7-6 (8-6), 1-6, 7-5 in the first round Monday during an afternoon that was hot but not particularly humid. Murray was mystified the cramps came on so early — at the start of the third set, after about 90 minutes on court.

“When it starts to kind of go everywhere, you don’t know exactly where it’s going to creep up next,” he said. “When you stretch one muscle, something else then cramps, too.”

It started in the back of his left shoulder, and spread to his forearm. The right-handed Murray couldn’t toss the ball high enough to get much pace on his serves.

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Between points, he would twist his body to awkwardly stretch his left side. After hitting a winner, he would reach for his quad.

Murray was twice down a break in the fourth set, but the 70th-ranked Haase unraveled with a string of unforced errors. He wasted three break points in the final game, when a missed call also cost him.

The eighth-seeded Murray had felt confident in his conditioning after productive training sessions in Miami, where he weathered far more heat and humidity. The 2012 U.S. Open champion wondered if something was amiss in his nutrition.

“Cramping in my left forearm?” a bewildered Murray said. “I mean, I didn’t use my left forearm a whole lot today.”

Murray is notorious for suddenly clutching at an ailment after a poorly played point. On this day, though, the misery was clearly real. The two-time major champion went after winners to shorten points, tried to stay upright to keep the strain off his legs. It was just enough to eke out the victory.

“I don’t think, if it would have gone to five sets, I would’ve been the favorite,” Murray said.

Fifth-seeded Maria Sharapova, who missed last year’s tournament because of an injured right shoulder, made a successful return by winning the final 10 games to beat Maria Kirilenko 6-4, 6-0 in a matchup of 27-year-old Russians and longtime friends.

Third-seeded Stan Wawrinka of Switzerland and fifth-seeded Milos Raonic of Canada both advanced in straight sets.

Top-seeded Novak Djokovic of Serbia played later under the lights at Arthur Ashe Stadium in the first night session of the year’s last Grand Slam tournament. Djokovic beat Diego Schwartzman of Argentina 6-1, 6-2, 6-4.

“I hope it was a midnight delight for all you guys,” Djokovic told fans in an on-court interview.

Earlier, American Venus Williams, 34, beat 43-year-old Kimiko Date-Krumm of Japan 2-6, 6-3, 6-3. The combined age of 77 was believed to be the oldest for a women’s Grand Slam pairing in the Open era, WTA officials said.

“According to Kimiko, I’ve got another decade,” Williams said.

Williams spent more than a minute during the match trying to get away from a persistent bee.

Along with the 19th-seeded Williams, 21st-seeded American Sloane Stephens also advanced by routing Annika Beck of Germany.

Second-seeded Simona Halep of Romania rallied from a set down for a 6-7 (2-7), 6-1, 6-2 victory over 20-year-old Danielle Rose Collins, an American who was playing her first main-draw match at a Tour-level event.

As a sophomore at Virginia, an unseeded Collins won the NCAA title to earn a wild card into the U.S. Open.