Nick Kyrgios of Australia receives treatment from an ATP trainer during an injury timeout Wednesday night. Kyrgios retired from the match one set down and trailing 3-0 in the second set to American Tennys Sandgren. (Geoff Burke/USA Today)

Play on Citi Open’s main stage ended with some drama Wednesday night when Nick Kyrgios, ranked 20th in the world and ever unpredictable, retired from his match against American Tennys Sandgren while trailing 6-3, 3-0, citing pain in his right shoulder.

“Just struggling, mentally, physically,” Kyrgios said in the hallway underneath stadium court after the crowd loudly booed his exit. “This was a tough one.”

Kyrgios had been recuperating from a hip injury that caused him to retire during his first-round match at Wimbledon and pull out of a tournament in Atlanta, where he was the defending champion. But the Australian said his hip felt fine during the match — he instead called the medical trainer out early in the first set to receive treatment after feeling pain in his shoulder, which he said hadn’t been sore or hurting before the Citi Open.

“My shoulder kind of just came on today, I hadn’t been feeling it,” Kyrgios said. “And my hip has been fine. I’ve been very cautious with it, I haven’t been training enough, at all, to play a tournament like this so. I just wanted to see how it goes, but obviously I’m far from ready. … I just hadn’t hit enough balls. I don’t know, obviously the match intensity is different in practice but my shoulder’s a little bit sore. Obviously with the U.S. swing coming up I don’t think it’s really worth playing through here and jeopardizing my U.S. Open.”

After calling the trainer, Kyrgios spent every changeover looking up towards the player box where his mother was sitting and shaking his head to indicate that he couldn’t continue. Sandgren, who notched his first-ever ATP tour win in Washington on Tuesday, continued to win games as the crowd at Rock Creek Park Tennis Center heckled Kyrgios.

One fan yelled “Show a little heart!” Another called the 22-year-old pathetic. Kyrgios said it wasn’t just his body that let him down. He’s been struggling mentally as well.

“Not great, honestly. Been struggling,” he said. “Obviously showed today. Just in general. It’s not motivation, just off-court issues, I guess.”

Sandgren, ranked 102nd in the world, now advances to face fifth-seeded Alexander Zverev, disrupting what would have been a juicy matchup of tennis’s rising stars.

Zverev defeated Australia’s Jordan Thompson 4-6, 6-3, 7-6 (7-5), earlier in the night in his first hard court match since a March tournament in Miami. The 20-year-old German, ranked eighth in the world, had been looking forward to playing Kyrgios as well.

“I think apart from the Big Four playing against each other, me against him [Kyrgios] or me against Dominic (Thiem) might be one of the biggest matchups in tennis for the media and social media and tennis fans,” he said.

Sandgren wasn’t the only spoiler Wednesday night. Citi Open defending champion Gael Monfils was also upset by qualifier Yuki Bhambri of India, 6-3, 4-6, 7-5, and Daniil Medvedev defeated No. 14 seed Steve Johnson 3-6, 6-4, 7-6 (7-1) in the final match of the evening, ending at 1:22 a.m.

The tournament’s early morning drama was sparse in matches during the day. Canada’s Milos Raonic needed two hours to down Frenchman Nicolas Mahut 7-6 (6-2), 7-6 (10-8) and ultimately leaned on his whiplash-inducing serve to advance. The 6-foot-5 slugger served a mountainous 26 aces and recorded the fastest-ever serve at the Citi Open at 151 mph.

Raonic hopes Washington is a turning point in his season. The Canadian was the champion here in 2014 and returned this year ranked 10th in the world and seeded third in the event, but he hasn’t won an ATP title in 2017. He has won at least one ATP title every year since 2011.

“It’s a big chance to turn things around,” Raonic said after.

His compatriot, Eugenie Bouchard, also a main draw on Stadium Court today, hopes the same. Bouchard is playing the Citi Open for the fifth time in her career and looked strong enough Wednesday to finally break through in Washington.

The 23-year-old has never advanced past the quarterfinals at the Citi Open and lost in the first round the past two times she’s visited Rock Creek Park Tennis Center, in 2013 and 2015. But she handled New Jersey native Christina McHale with ease in her opening round match, a 7-6 (8-6), 6-0 win under a hot mid-afternoon sun. She will face Andrea Petkovic of Germany in the second round.

Three years after Bouchard’s breakout season, in which she reached the semifinals at the Australian Open and French Open, lost in the finals at Wimbledon and climbed to a career-best No. 5 world ranking, the Canadian has been toiling to unlock her best tennis once again. She spent some time with Andre Agassi in Las Vegas this summer picking up pointers from the legendary champion and did a bit of mental re-calibration before starting her summer swing.

“I feel like I’m a work in progress,” Bouchard said after she and Sloane Stephens won their doubles match, 2-6, 6-3, 10-6, following her singles win. “Just trying to work a lot on things in the game — serve, return, playing my aggressive style of game and have the right mentality while doing it, be mentally strong during matches.”

Bouchard is unseeded in Washington and entered the week ranked No. 73 in the world. She exited in the first round of the three grass-court tournaments she played this summer and has dealt with a right ankle injury toward the end of May but hopes to revive her season on hard courts. A win Wednesday was a sure-footed step in the right direction.

Samantha Pell contributed to this report.

More Citi Open:

For players at the Citi Open, there’s an art to choosing practice partners

For Steve Johnson, tennis is no escape from grief. Sometimes, it’s the opposite.

Tuesday: Dominic Thiem cruises; Kei Nishikori outlasts Donald Young

Tuesday: Following three-hour rain delay, Jack Sock ousts Marius Copil

With past four champions back, the men’s field is loaded with talent