LONDON, June 30 (Reuters) - Wimbledon organisers on Wednesday defended the state of the grasscourts after concerns from players that the lawns are too slippery. Seven-times champion Serena Williams was forced to retire from her first-round match on Tuesday after appearing to slip and injuring her leg in the opening set against Belarussian Aliaksandra Sasnovich on Centre Court. Frenchman Adrian Mannarino also quit at two sets apiece against eight-times champion Roger Federer after sliding over on Centre Court and injuring his knee. Men"s top seed Novak Djokovic fell twice in the first set of his first-round match against Jack Draper on Monday while twice champion Andy Murray said he felt that Centre Court was slippery with the sliding roof closed. The opening two days of the Championships have been wet with play disrupted on uncovered outside courts while the Centre Court and Court One roofs have been well-used. The All England Club said the courts, which were unused last year after the tournament was cancelled because of the coronavirus pandemic, have been prepared in the usual meticulous way by the groundstaff. "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 club said in a statement. "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. 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. "With each match that is played, the courts will continue to firm up." Wimbledon"s lawns are prepared using state-of-the-art turf technology with measurements taken every morning for hardness and moisture content. Even under the roof, the advanced air conditioning system removes moisture from the air to prevent the surface sweating. Federer said he was asked by the referee whether he felt Centre Court was more slippery than usual. "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 first two matches are always extremely difficult. But it"s always been like this. As the tournament progresses, usually it gets harder and easier to move on."