LONDON — The screams were startling.

Bethanie Mattek-Sands, a 32-year-old American who came to Wimbledon eyeing a fourth consecutive Grand Slam doubles title, fell to the grass when her right knee buckled as she moved toward the net in a second-round singles match Thursday.

She immediately clutched her knee and, down on the turf, wailed loudly, imploring for someone to “Help me! Help me!”

Her opponent, Sorana Cirstea, immediately climbed over the net to check on Mattek-Sands, who after about 20 minutes was removed from Court 17 on a stretcher and taken to a hospital.

“Her knee was in a very weird position. I’ve never seen anything like this, probably, except in the movies. And, yeah, I panicked a little bit, as well,” Cirstea said. “Then I called for help, but no one was coming. Then tried to comfort her as much as I could. But, I mean, you could feel the pain.”

The extent of Mattek-Sands’ injury, which came in the third set’s opening game, was not immediately known. But word quickly spread around the grounds, generating concern among players. She’s popular on tour, known for her gregarious personality, loud laugh and original fashion choices, including the stars-and-stripes knee-high socks she wore while teaming with Jack Sock to win a mixed-doubles gold medal for the U.S. at last year’s Rio Olympics.

“It’s the peak of her career right now,” said Bob Bryan, whose twin brother Mike won the 2015 French Open mixed-doubles title with Mattek-Sands. “She’s a fun-loving girl. She doesn’t have any enemies in the locker room.”

She’s also quite a doubles player, ranked No. 1 right now after teaming with Lucie Safarova of the Czech Republic to win the past three major championships and a total of five. Safarova heard about Mattek-Sands on TV and ran to her court, then stood nearby and wiped away tears as she saw her friend and playing partner in distress.

“Just terrible what happened,” Safarova said, her voice barely above a whisper, after losing her singles match to Shelby Rogers of the U.S. in three sets. “Obviously, I’m very sad for her. Doesn’t matter about whatever goals we had. It’s just about her being healthy.”

Mattek-Sands — who was born in Minnesota, lived in Wisconsin and now calls Arizona home with her husband Justin — thought about quitting tennis years ago after a series of injuries. There was hip surgery less than a week after her wedding in late 2008, a torn shoulder in 2011, a broken right big toe in 2012. Her ranking dropped outside the top 250 in singles and doubles in 2014, when she missed six months after another hip operation.

“I’m just, like, really hurt for her,” Safarova said.

There was no indication that the condition of the grass on the court had anything to do with Mattek-Sands’ fall, but playing surfaces around the All England Club were a source of complaints by others Thursday. In particular, the areas near many baselines are brown and worn, with little or no grass in spots — looking the way Wimbledon’s courts usually do by late in Week 2, not as soon as Day 4 — something players said they were told was a result of unusual heat and lack of rain in recent weeks.

“The patch near the baseline is eaten up and the dirt underneath is like ice. Look around, people are going down left and right,” said 46th-ranked Alison Riske of the U.S., whose 2-6, 6-4, 6-4 victory over 12th-seeded Kristina Mladenovic of France at Court 18 was one of a handful of upsets in the women’s draw, including No. 3 Karolina Pliskova’s three-set loss to Magdalena Rybarikova.

“I was worried about, maybe, our safety, to be honest,” Riske said. “The court was pretty slippery.”

Both she and Mladenovic complained to the chair umpire before play started, and again after each took a tumble in the first two games.

“I did feel strongly about it, but I knew in my mind: Where else are they going to put us and is it going to be better? Kiki and I had a conversation,” Riske said, referring to Mladenovic by her nickname. “And I said, ‘Look, Kiki, are the other courts going to be any different?’ And obviously, they’re not.”

Mladenovic said she twisted her ankle during the warmup period and said she’ll have an MRI on her right knee, which turned awkwardly in the second game.

“There’s no grass. I don’t know how to describe it,” said Mladenovic, who held a bag of ice on her swollen knee.