Title: List of Grand Slam women's doubles champions
Content:  Note: players who won 4 titles in a season are not included here
Title: Grand Slam (tennis)
Content:  at the same time achieve the Grand Slam'." As of 2012, however, the ambiguity was resolved, with the ITF's current constitution stating "The Grand Slam titles are the championships of Australia, France, the United States of America and Wimbledon. Players who hold all four of these titles in one calendar year achieve the 'Grand Slam'." Combining the Grand Slam and the non-calendar-year Grand Slam, only eight singles players on 11 occasions achieved the feat of being the reigning champion of all four majors, three men (Don Budge, Rod Laver, Novak Djokovic) and five women (Maureen Connolly, Margaret Court, Martina Navratilova, Steffi Graf, Serena Williams).
Title: 21st century
Content: Roger Federer won 20 Grand Slam titles (6 Australian Opens, 1 French Open, 8 Wimbledons, and 5 US Opens) to surpass Pete Sampras' record of 14. ; Roger Federer, Rafael Nadal and Novak Djokovic each completed a Career Grand Slam, winning the singles championships in the Australian Open, French Open, Wimbledon and US Open; Nadal also won the Olympic Singles gold medal in the 2008 Beijing Summer Olympics to complete a Golden Career Slam. ; At the 2010 Wimbledon Championships, John Isner and Nicolas Mahut completed the longest tennis match ever. Isner won 6–4, 3–6, 6–7(7), 7–6(3), 70–68. ; In 2019, Rafael Nadal became the first male player to win a single Grand Slam tournament (French Open) 12 times. 
Title: 21st century
Content: Serena Williams won 23 Grand Slam titles (7 Australian Opens, 3 French Opens, 7 Wimbledons, and 6 US Opens) in the 21st century, to add to her 1999 US Open title. ; Maria Sharapova became the first female Russian player to reach No.1 on August 22, 2005. She also retired in 2020. ; China's Li Na won the 2011 French Open, becoming the first player, male or female, from that country to win a Grand Slam. ; Belarusian Victoria Azarenka won the 2012 Australian Open, becoming the first player, male or female, from that country to win a Grand Slam, and also hold the No.1 ranking (taking over from Caroline Wozniacki). 
Title: Grand Slam (tennis)
Content:  The career achievement of winning all four major championships in one discipline is termed a "Career Grand Slam". In singles, eight men (Fred Perry, Don Budge, Rod Laver, Roy Emerson, Andre Agassi, Roger Federer, Rafael Nadal and Novak Djokovic) and ten women (Maureen Connolly, Doris Hart, Shirley Fry Irvin, Margaret Court, Billie Jean King, Chris Evert, Martina Navratilova, Steffi Graf, Serena Williams and Maria Sharapova) have completed a Career Grand Slam. Three men (Emerson, Laver and Djokovic) and five women (Court, Evert, Navratilova, Graf, Williams) managed to achieve the feat more than once over the course of their careers. Only six players have completed a Career Grand Slam in both singles and doubles: one male (Roy Emerson) and five female (Margaret Court, Doris Hart, Shirley Fry Irvin, Martina Navratilova, and Serena Williams). Court, Hart and Navratilova are the only ones to have completed a "Career Boxed Set", that is, winning all four major titles in singles, doubles, and mixed doubles; this has never been done by a male player.
Title: List of Grand Slam men's singles champions
Content: } 
Title: List of Grand Slam men's doubles champions
Content:  Note: players who won 4 titles in a season are not included here.
Title: Serena Williams
Content:  is the only tennis player, male or female, to have won three of the four Grand Slams at least 6 times. Williams is also a five-time winner of the WTA Tour Championships in the singles division. Williams has won 14 Grand Slam doubles titles, all with her sister Venus, and the pair are unbeaten in Grand Slam doubles finals. As a team, she and Venus have the third most women's doubles Grand Slam titles, behind the 18 titles of Natasha Zvereva (14 with Gigi Fernández) and the record 20 titles won by Martina Navratilova and Pam Shriver. She and Venus are the only tennis players in history with four Olympic gold medals, as 
Title: List of Grand Slam women's doubles champions
Content:  List of Women's Doubles Grand Slam tennis tournament champions: The might be pairing to complete the "Grand Slam" is the team of Martina Navratilova and Pam Shriver in 1984, and their eight consecutive slam win streak still stands as the all-time record. Maria Bueno in 1960 and Martina Hingis in 1998 won the yearly grand slam with various partners in the slams. Four players have completed a career doubles golden-slam by winning a gold medal at the Olympics and all four Majors during their respective careers: Venus Williams and Serena Williams paired together, and individually Pam Shriver and Gigi Fernández.
Title: List of Grand Slam singles champions by country
Content:  This is an all-time list of winners of the four Grand Slam men's and women's singles tennis tournaments, organized by country. The year of the first win in each tournament is shown in parentheses. Each player's first grand slam tournament win is shown in bold. The greatest number of wins in each country (in the Total column) is shown in bold.

Question: who has won more grand slam titles in tennis?
Answer: Roger Federer.