There have been 29 men who have been ranked World No. 1 since the rankings began in 1973, but Guillermo Vilas one star can be considered desperately unlikely not to be included in that.
Vilas was a dominant force, particularly on clay, during an illustrious career that saw him win 62 titles between 1968 and 1992.
The Argentine hit a particularly strong vein of form between 1977 and 1979, when he won two Australian Open titles, a French Open, and a US Open.
Despite four Grand Slams in 20 months, the star never became World No. 1 despite appearing to be eligible for the crown.
Why Guillermo Vilas was never given World No. 1 status
Vilas was a tour-de-force on the ATP Tour in the late 1970s, and 1977 saw him dominate the likes of Bjorn Borg and Jimmy Connors.
In 1977, the star won 16 ATP singles titles and went on a 53-match winning streak on clay, but he only reached as high as World No. 2.
The old ranking system was flawed, and there was debate about whether the Argentine actually deserved to reach the summit of the ATP Tour.
After digging into the data, Argentinian journalist Eduardo Puppo and Romanian mathematician Marian Ciulpan concluded that Vilas should have been World No. 1 between 1975 and 1976.
However, the ATP Tour is still yet to recognise that Vilas was ever World No. 1, making him arguably the most successful player in history to never get the top spot.
In 2020, Netflix released a documentary about Vilas potentially deserving the World No. 1 spot, called Guillermo Vilas: Settling the Score.
Players to win multiple Grand Slams without becoming World No. 1
Vilas is not the only star to win more than one Grand Slam without reaching the top of the ATP Tour rankings.
Most famously in the modern era, Stanislas Wawrinka picked up an Australian Open, a US Open, and a Roland Garros without becoming World No. 1.
The Swiss star reached a career high of World No. 3 in 2014, which was actually before he had won his last two Grand Slams.
On the WTA Tour, Hana Mandlíková won two Australian Open titles and a US Open crown and only reached World No. 3.
