Caroline Wozniacki R (Danish: [kʰɑʁoˈliːnə vʌsniˈɑkʰi]; born 11 July 1990) is a Danish inactive professional tennis player. She has been ranked as the world No. 1 in women's singles by the WTA, holding the position for a total of 71 weeks (including as the year-end world No. 1 in 2010 and 2011). Wozniacki has won 30 WTA Tour-level singles titles, including a major at the 2018 Australian Open as well as the 2017 WTA Finals.

Wozniacki had a successful junior career, winning the junior title at the 2006 Wimbledon Championships. After being named the WTA Newcomer of the Year in 2008, she contested two major finals at the 2009 and 2014 US Opens, as well as the final at the 2010 WTA Tour Championships, becoming the world No. 1 in 2010. Wozniacki retired from the sport following the 2020 Australian Open. In 2022, she became a commentator for Tennis Channel and ESPN, before beginning a professional comeback at the 2023 Canadian Open and the US Open. She is known for her footwork and defensive skill.

Wozniacki was born in Odense, Denmark. She is of Polish descent. Her mother Anna played on the Polish women's national volleyball team, and her father Piotr played professional football. The couple moved to Denmark when Piotr signed for the Danish football club B1909. Her older brother, Patrik Wozniacki, is a former professional footballer in Denmark.

Wozniacki had a career-high junior ranking of No. 2 in the world. She made her debut on the ITF Junior Circuit, the premier 18-and-under junior tour that is run by the International Tennis Federation (ITF), in October 2003 at 13 years and 3 months old. During her first month, she entered two Grade 5 events in Denmark, the lowest-level tournament on the tour. Wozniacki reached the final at both singles events, winning the second over compatriot Hanne Skak Jensen. With this success, she moved up to Grade 3 and Grade 4 tournaments, winning her first four singles events of 2004. Her win streak of five tournaments and 27 matches came to an end in April with a semifinal loss to Latisha Chan at the International Juniors Championships in Manila, her first event outside of Scandinavia as well as her first Grade 1 event. From this point on, Wozniacki only competed in Grade A and Grade 1 events, the two highest-level tournaments. She made her Grade A debut at Wimbledon before turning 14, losing her first main draw match to Bojana Bobusic. Towards the end of the year, Wozniacki won her first Grade A title at the Osaka Mayor's Cup.

Wozniacki won several junior tournaments in 2005, including the Orange Bowl. She made her debut on the WTA Tour at Cincinnati's Western & Southern Open on 19 July 2005, losing to the top-seeded and eventual champion Patty Schnyder in the first round. In the Nordic Light Open, her other WTA tournament of the year, she lost to Martina Suchá in the first round.

In 2006, she was the top seed at the Australian Open (junior girls' singles), but lost the final to eighth-seeded Anastasia Pavlyuchenkova. She was seeded second with partner Anna Tatishvili in the doubles tournament, but the pair was knocked out in the semifinals by the French-Italian pair of Alizé Cornet and Corinna Dentoni, who were seeded eighth. In February at the Memphis, she reached her first WTA Tour quarterfinal, beating Kristina Brandi and Ashley Harkleroad before losing to third-seeded Sofia Arvidsson of Sweden. Before Wimbledon Wozniacki won the Liverpool International Tennis Tournament, an exhibition tournament, beating Ashley Harkleroad in the final. Later that year she was given a wild card to the qualifying draw at Wimbledon, where she was beaten in the first round by Miho Saeki. However, Wozniacki went on to win the Wimbledon girls' singles title, beating Magdaléna Rybáriková in the final. In August, she reached another WTA Tour quarterfinal, this time at the Nordic Light Open in Stockholm. She defeated top-100 players Iveta Benešová and Eleni Daniilidou, then lost to eventual champion and third-seeded Zheng Jie.

Wozniacki was seeded second in the US Open girls' singles. In the first round, she won the first set against Russian Alexandra Panova, but was disqualified in the second set for verbally abusing an umpire. Wozniacki was said to have used an expletive in referring to a linesman who made a disputed call. However, on her blog, she claimed to have said "take your sunglasses of [sic]" and was mistaken for talking to the linesman, when she in fact was criticizing herself after the next point.

In her last junior tournament, the Osaka Mayor's Cup, she won both the girls' singles and doubles. Her first title on the senior tour came in October when she won a $25k tournament in Istanbul by beating Tatjana Malek in the final. Wozniacki was set to face Venus Williams on 27 November in an exhibition match in Copenhagen, but five days before the event, Williams cancelled because of an injury. The two did, however, face each other in the Memphis WTA Tier III event on 20 February. Williams beat Wozniacki, ending a nine-match winning streak for Wozniacki. On 29 November Wozniacki was named ambassador for Danish Junior Tennis by the Culture Minister of Denmark at the time, Brian Mikkelsen.