WIMBLEDON, England — Matches continued on Centre Court at Wimbledon as rain fell outside on the first two days of the tournament that showcases top stars in an arena considered a cathedral of the sport before thousands of fans.

And while the rain wasn't falling inside Centre Court, the players were. At their best, fast, low-bouncing grass courts encourage engaging, all-court tennis that rewards risk and punishes passivity.

At their worst, the courts' slick surfaces allow flat-soled tennis shoes to skid and send players falling to the ground, often in pain.

Slippery conditions caused injuries in back-to-back matches on Tuesday, affecting the fortunes of two of the sport's most venerated stars. First, Roger Federer advanced after his opponent, Adrian Mannarino, slipped and sustained a knee injury while leading two sets to one.

In the match that followed, the seven-time Wimbledon champion Serena Williams slipped and aggravated a hamstring injury, forcing her to stop her first-round match against Aliaksandra Sasnovich after only six games. The injury saw Williams abandon what many considered one of her last best chances to win an elusive 24th major title.

Federer was in his postmatch news conference when he was informed of Williams's injury. He let out an exasperated, rueful, "Come on."

"This is obviously terrible, that it's back-to-back matches and it hits Serena as well," he said. "Oh, my God, I can't believe it."

Williams and Mannarino were the only players forced to retire during the first two days of the tournament, suggesting that conditions on Centre Court were considerably more treacherous than elsewhere on the All England Club's 17 other courts.

Sasnovich, Williams's opponent, said the court was so slippery that she didn't run to retrieve wide-angled shots as she normally would. Williams's one-time mixed doubles partner Andy Murray, who played on the court a day earlier, said the surface made moving around difficult.

Grass courts, which originated as the predominant surface in tennis during Victorian times, are now an anachronism reserved only for Wimbledon and a small number of tournaments around it on the calendar. The Australian Open and the U.S. Open, both of which used to be held on grass, switched to hard courts decades ago.

But while grass court tennis is considered traditional — the sport was originally called lawn tennis, after all — Wimbledon's offering of indoor grass court tennis is newfangled. Wimbledon added its retractable roof to Centre Court only in 2009, and a roof to the secondary No. 1 Court in 2019. An extensive ventilation system was installed along with the roof, but the grass remains dewy.

"I do feel it feels a tad more slippery, maybe, under the roof," Federer said. "I don't know if it's just a gut feeling. You do have to move very, very carefully out there. If you push too hard in the wrong moments, you do go down."

The frequent rain on Monday and Tuesday both dampened the court before the roof closed and kept the lush, untouched grass from drying out as quickly as it normally would in open air and wind.

Before the tournament began, the only action Centre Court had seen all year was a gentle doubles match played by four members of the All England Club on Saturday.

In a statement issued on Tuesday evening, the All England Club defended the condition of its courts.

"The preparation of the grass courts has been to exactly the same meticulous standard as in previous years," the club said. "Each grass court is checked by the Grand Slam Supervisors, Referee's Office and Grounds team ahead of play commencing, and on both days of the Fortnight they have been happy with the conditions and cleared the courts for play.

"The weather conditions on the opening two days have been the wettest we have experienced in almost a decade, which has required the roof to be closed on Centre Court and No. 1 Court for long periods," the statement continued. "This is at a time when the grass plant is at its most lush and green, which does result in additional moisture on what is a natural surface."

The club added that "with each match that is played, the courts will continue to firm up."

Before the tournament, Novak Djokovic remarked on Saturday about what an honor it was to walk onto the "virgin grass" of Centre Court, as the men's singles defending champion is given the honor of playing the stadium's opening match on the first day of the tournament each year. In his four-set win over Jack Draper on Monday, Djokovic awkwardly fell to the ground several times when trying to change directions or follow through on his strokes.

"To be honest, I don't remember falling this many times on the court," Djokovic said in his on-court interview after the match. "It's quite slippery. Whether it's because the roof was closed or it was raining quite a lot in the last few days, I don't know."

"I'm going to work on my movement and slide a bit less on grass," Djokovic added with a self-deprecating laugh. "It seems to be not working as well on this surface."

Coco Gauff, the teenage phenom who could have faced her idol Serena Williams for the first time if both had advanced to the fourth round pointed out that the scarcity of grass court tennis on the calendar gives players little chance to master the movement needed to ensure firm footing — especially in this year, as the entire grass court swing was canceled last year because of the coronavirus pandemic.

"Just in general, no one is really used to moving on grass because the season is so short," Gauff said. "People are bound to have slips and falls."

This is not the first edition of Wimbledon plagued by falls. In 2013, stars including Maria Sharapova, Victoria Azarenka and Caroline Wozniacki were injured after slipping on the grass. The blame was put on a particularly damp spring and humidity.

Grass courts at other events outside Wimbledon have also proved injurious in recent years. At Queen's Club in London two years ago, Juan Martín del Potro slipped on slick grass and sustained a knee injury requiring multiple surgeries that have kept him from returning to competition.

In Halle, Germany, this month, the 16th-ranked David Goffin slipped and fell when trying to plant his feet on the grass during a point, sustaining an injury that kept him out of Wimbledon.

Player safety is a concern to which tennis authorities have not always seemed especially responsive. At the Italian Open in Rome last month, Rafael Nadal tripped over a line that had come unpinned from the clay court. At the Australian Open, tournament officials have been slow to stop play during both extreme heat and poor air quality resulting from nearby wildfires.

Rebound Ace, a rubbery synthetic hardcourt surface made in part from old car tires, which was used at the Australian Open from 1988 to 2007, was considered by players to be too sticky in hot weather. They blamed it for injuries.

At the 2011 U.S. Open, heavy rain from Hurricane Irene on the eve of the tournament caused a leak to spring from a crack in the hardcourt surface of the secondary Louis Armstrong Stadium, stopping play on that court for the rest of the tournament.

Andy Roddick, who was playing David Ferrer on that court when play was suspended, led players, officials and fans on a march to the smaller but dry Court 13.

The most rapid changes to a surface based on player feedback were made at the 2012 Madrid Open, where the clay was dyed a bright cerulean instead of its normal rust color. Though it was telegenic and there were no serious injuries on the surface, the blue clay was dropped the next year after players including Nadal said they would not play on it again. The next year, the red clay was back.

Wimbledon, which treasures tradition, is unlikely to reconsider its hallmark green grass, even after it sent one of the sport's biggest stars tumbling out of the tournament.

"Our long-serving Grounds team have experienced nearly every combination of weather conditions possible," the club's statement said Tuesday. "They keep abreast of and utilize the latest grass court technologies, prepare for every weather eventuality and react to the current conditions on a daily basis. We will continue to monitor these readings and adjust our care plan for the grass appropriately."