NEW YORK — If this is how Serena Williams serves when she can’t practice properly because her right shoulder is sore, watch out when she’s 100 percent healthy.

A year after falling two wins short of a calendar-year Grand Slam by bowing out in the U.S. Open semifinals, Williams showed zero signs of shoulder trouble Tuesday night as she began her bid for a record-breaking 23rd major title.

She hit 12 aces and reached 121 mph on her powerful serve during a 6-3, 6-3 victory over Ekaterina Makarova, a potentially tricky first-round opponent at Flushing Meadows.

“I was pleased with my serve, because I haven’t been hitting a lot of serves at all,” the 34-year-old Williams said. “In practice, none of them were going in, so I was definitely excited about that.”

Since equaling Steffi Graf’s mark for most Grand Slam singles trophies in the Open era, which dates to 1968, by earning No. 22 at Wimbledon in July, the No. 1-ranked Williams had only entered one event – the Rio de Janeiro Olympics, where she was upset in the third round. She cited a sore shoulder in withdrawing from a hard-court tournament a week later.

She looked perfectly fine against Makarova, a two-time Grand Slam semifinalist who is ranked 29th and beat Williams in straight sets at the 2012 Australian Open.

“I knew today I needed to be focused because I’ve played her. She’s gotten to the semifinals. She goes deep in majors. She knows how to play big matches on big courts. She’s not intimidated,” Williams said. “I knew I had to really come out today. It was my only option, really.”

Well, consider that done.

Wearing black sleeves on each arm – she called the accessories “definitely functional” because they “keep my muscles warm” – Williams averaged 108 mph on first serves and won 17 of the first 20 points she served, 36 of 46 overall, never appearing to be the least bit bothered by anything.

And she even figured maybe the time she couldn’t spend serving during training sessions paid off, in a way, because she was forced to work on other aspects of her game, including footwork.

“I couldn’t hit any balls. I wanted to stay fit, so … I guess that kind of helped me out a little bit,” she said.

In other matches Tuesday, Eugenie Bouchard lost 6-3, 3-6, 6-2 to 72nd-ranked Katerina Siniakova of the Czech Republic, a player who only once has been as far as the third round at a major tournament. The match was filled with 46 unforced errors by Bouchard, who also was treated for blisters on her feet. It represented the latest early loss for a 22-year-old Canadian who reached three Grand Slam semifinals two years ago – and none since.

A year ago at Flushing Meadows, Bouchard got a concussion from a fall at the facility and withdrew before playing in the fourth round, and then missed most of the rest of the season. She filed suit against the U.S. Tennis Association in U.S. District Court in Brooklyn in October, and that case is still pending.

When she was at her peak, an early major loss by Bouchard was rather newsworthy. Her up-and-down 2015 and 2016 have changed that.

In other first-round action, Andy Murray beat Lukas Rosol 6-3, 6-2, 6-2; and Williams’ sister, Venus, got through a tougher-than-expected 6-2, 5-7, 6-4 win against Kateryna Kozlova.

There were various upsets during the afternoon, including 19-year-old American Jared Donaldson’s 4-6, 7-5, 6-4, 6-0 elimination of 12th-seeded David Goffin, and a loss by No. 29 Sam Querrey. Three seeded women departed, including former No. 1 and 2008 French Open champion Ana Ivanovic.