NEW YORK — Eugenie Bouchard wrung her hands at her post-match news conference. She rubbed her lower lip. She squeezed her left arm.

While her body language screamed discomfort Tuesday, when the main topic of discussion was Bouchard’s ongoing lawsuit against the US Open rather than her first-round loss, her words were measured. The once rising star answered every question.

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 US Tennis Association in US District Court in Brooklyn in October, and that case is still pending, putting the 2014 Wimbledon runner-up in the odd position of competing this week at an event whose organizers she is suing.

‘‘If I sit down and think about it, yeah, it’s definitely a strange situation. But it’s something that’s so far in the back of my mind. I don’t think about it on a daily basis, at all. I have people, lawyers, working on that side of it,’’ Bouchard said. ‘‘So it’s really not something I think about much at all. Obviously, being here, it’s crossed my mind. But besides that, I mean, it has nothing to do with my day-to-day life.’’

Her 6-3, 3-6, 6-2 exit against 72d-ranked Katerina Siniakova of the Czech Republic, a player who only once has been as far as the third round at a major tournament, 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.

In other first-round action on Day 2 at the year’s last Grand Slam tournament, Serena Williams started her bid for a record-breaking 23d major title by showing zero signs of trouble from a right shoulder she hass said was sore, hitting 12 aces in a 6-3, 6-3 victory over Ekaterina Makarova. Earlier, Williams’s sister Venus got through a tougher-than-expected 6-2, 5-7, 6-4 win against Kateryna Kozlova.

Bouchard lost 14 of 17 matches leading into the 2015 US Open but, after working a bit with Jimmy Connors, appeared to be back on the upswing in New York by reaching the fourth round. Then came her slip-and-tumble, and the concussion diagnosis, and she pulled out of what would have been a matchup against eventual runner-up Roberta Vinci.