The 2015 US Open was a tennis tournament played on outdoor hard courts. It was the 135th edition of the US Open, the fourth and final Grand Slam event of the year. It took place at the USTA Billie Jean King National Tennis Center. Marin Čilić was the defending champion in the men's singles event, but lost to eventual champion Novak Djokovic in the semifinals. Serena Williams was the three-time defending champion in the women's singles event and was also trying to complete the calendar Grand Slam since Steffi Graf in 1988, having won the Australian Open, French Open and Wimbledon, but lost to Roberta Vinci in the semifinals. Flavia Pennetta won the women's singles title and became the first Italian to win the US Open. The finalists Vinci and Flavia Pennetta were childhood friends from Southern Italy and grew up together. Tournament The 2015 US Open was the 135th edition of the tournament and it was held at the USTA Billie Jean King National Tennis Center in Flushing Meadows–Corona Park of Queens in New York City, New York, United States. The tournament was an event run by the International Tennis Federation (ITF) and is part of the 2015 ATP World Tour and the 2015 WTA Tour calendars under the Grand Slam category. The tournament consists of both men's and women's singles and doubles draws as well as a mixed doubles event. There are singles and doubles events for both boys and girls (players under 18), which is part of the Grade A category of tournaments, and singles, doubles and quad events for men's and women's wheelchair tennis players as part of the NEC tour under the Grand Slam category. In addition, the annual men's and women's Champions Invitational doubles events were held, with eight male and eight female former Grand Slam champions taking part. For the second year running, the American Collegiate Invitational competitions is organized, where top sixteen American collegiate players compete in men's and women's singles events. Exhibition matches also take place. The tournament is played on hard courts and takes place over a series of 17 courts with DecoTurf surface, including the three main showcourts – Arthur Ashe Stadium (with permanent steel erected and new video boards, LED court lighting and sound system in place, as part of a refurbishment project), Louis Armstrong Stadium and Grandstand. It is the last ever US Open played on courts