Ellen Jones, Cardiff Metropolitan University and Robert Meyers, Cardiff Metropolitan UniversitySerena Williams has added to her grand slam singles tally at Wimbledon. Claiming the Venus Rosewater Dish for the sixth time on the famous centre court is her 21st major singles title and arguably makes her the greatest female player of all time.Comparing great champions from different eras is fraught with problems but still a conversation people love to have. The quality and depth of the competition, pressures and distractions off the court, advances in equipment and training techniques are among a long list of contributing factors that shape a player’s career.So, who is the greatest of all time? When the debate about “the best” occurs, four names are commonly discussed; Margaret Court, Martina Navratilova, Steffi Graf and Williams.Margaret Court Margaret Court in action. Bert Verhoeff, CC BY-SA Margaret Court’s career stats are unrivalled. Court dominated women’s tennis in the 1960s, boasting the highest-ever percentage career wins at 91.7%, winning 24 grand slam singles titles and 38 grand slam doubles titles. Court won most of her grand slam titles before the open era and at a time when not all of the best players competed at the Australian Open where she won 11 titles. It is frequently argued that, during her reign at the top, the competition did not have the depth or quality of the open era.Martina Navratilova Navratilova in 2001. Adrian Dennis/EPA Martina Navratilova’s professional career spanned 34 years, she amassed 59 grand slam titles including a record nine Wimbledon singles titles. She won an unprecedented 1,177 singles matches, a winning percentage of 86.8%. Martina has won more singles titles than any other player of the open era an amazing 167 titles.Her great rivalry with Chris Evert spanned most the of 70s and 80s with Navratilova coming out on top by 43 wins to 37. The contrast in their game styles, their different on-court demeanours and the fact that they played each other an incredible 80 times resulted in one of the greatest sporting rivalries of all time.Steffi Graf Graf in 1999. Anja Niedringhaus/EPA Steffi Graf won an incredible 22 grand slam singles titles in the open era. In 1998 she became the only player in history to win a calendar “golden slam” winning all four grand slam events and the Seoul Olympic gold. Graf holds the record for the total number of weeks at number one in the world of any player, male or female with an unprecedented 377 weeks – 187 weeks were consecutive, another record.Graf’s game style and athleticism resulted in an unrivalled versatility across all playing surfaces, winning each of the four majors at least four times. Graf is widely considered the greatest tennis player of all time and with those stats it is difficult to argue against it.The only question mark that can be raised about Graf’s record-breaking career was the impact of Monica Seles’ horrific stabbing that kept her out of the sport for two years. Seles had won eight of the 11 grand slam singles she had played before that day and had been world number one between 1991 and 1993. Graf had only won 11 of her major titles before the attack and went on to win six of the next ten slams that Seles missed while recovering from her injuries. Some may argue that as a result, a rivalry to match Evert-Navratilova was stolen from the game – and Graf went on to dominate women’s tennis.Serena WilliamsSerena Williams has taken women’s tennis to the next level. Like the greats that have come before her, she has raised the bar with the athleticism and power she brings to the game, her serve is arguably the greatest weapon the women’s game has seen. Her career so far has seen her win 36 grand slam titles, top the world rankings for a total of 247 weeks and win a career golden slam.Williams is continuing to dominate in her mid 30s and shows no signs of slowing down. In the open era her 20 grand slam singles titles is only surpassed by Graf, however, Serena’s career singles titles tally at 67 is considerably short of Navratilova’s 177 and Graf’s 107. Just one of 35 titles. Jonathan Brady Although she still has a way to go to match some the other great player’s stats, her dominance over her current rivals is unarguable. Challengers such as Maria Sharapova, Victoria Azarenka, Kim Clijsters, Martina Hingis and her own sister Venus Williams have all been well beaten over her career.Probably Serena Williams' main rival, Justine Henin, retired from the game in 2008 still ranked as world number one. After two years Justine returned to the game before her second retirement due to an elbow injury in 2011, leaving a 4-3 career winning record against Williams in grand slam events. Since Henin’s retirement, no other female player has looked to truly challenge Serena’s reign and maybe a great rivalry will be the only thing missing from her career.The road to greatnessTo compare these greats is problematic. It is not Margaret Court’s fault not all her competitors travelled to the Australian Open, it does not seem right to diminish Graf’s records due to Monica Seles' forced absence from the game or to penalise Serena for a lack of a true rival – a player can only beat who is in front of them to make their mark in history.Can Serena continue to dominate and surpass all records? Even if she does not, can she be considered the best? It is likely we will not have to debate these issues for much longer, if she stays healthy, is clever with her scheduling and continues to be hungry for success there will be no stopping her, and the question of who is the greatest will be answered emphatically.Ellen Jones is Lecturer in Performance Sport (Tennis), Performance Director of Tennis at Cardiff Metropolitan University.Robert Meyers is Senior Lecturer in Strength and Conditioning at Cardiff Metropolitan University.This article was originally published on The Conversation. Read the original article.