If Serena Williams wins the US Open on Saturday, she will have won all four majors in one calendar year, and thus completed the Grand Slam.
The Grand Slam is tennis's holy grail, and ahead of Serena's semi-final against Roberta Vinci today, we look at just how big an achievement it is.
The calendar Grand Slam has been achieved six times in tennis history.
On the women's side, Maureen Connolly (1953), Margaret Court (1970) and Steffi Graf (1988) have pulled the feat off.
On the men's side Don Budge (1938) and Rod Laver (1962 and 1969) have done it.
Without wishing to diminish the fantastic achievements of the above players, there are a few asterisks to be placed next to their names.
In the case of Budge and Connolly, tennis was a very different sport to what it was now, with the pool of players and competing nations considerably smaller. Moreover many of the players didn't travel to Australia for the Australian Open, thereby weakening the field.
That was still the case even at the time of Court and Laver, and in both of their cases, the four grand slams were played on two surfaces (Wimbledon, the US Open and Australian Open were all on grass, with the French on clay), whereas by the time Graf pulled off the feat, they were played on three - as of course they are now (with the US and Australian Opens moving to hard courts).
Laver is the only player to have won the Grand Slam twice, and he did it both as an amateur in 1962 and then seven years later as a professional in 1969 after the advent of the sport's 'open era'. He might well have pulled the feat off in the intervening years but as a professional was ineligible to enter the grand slams between 1963 and 1967.
Graf is not only the sole player to have completed the Grand Slam with the tournaments played on three different surfaces, she is also unique in having won the 'Golden Grand Slam', which includes winning Olympic Gold (in Graf's case in Seoul) as well as the four Majors in the same year.
Serena Williams would have to wait until 2016 and the Rio Olympics to pull that off.
He has not (the closest he has come was winning all but the French Open in 2006 and 2007). Rafael Nadal has also won three out of four in a calendar year (in 2010 when the Australian Open was the only major to elude him).
A quick glance at the players to have not won the Grand Slam perhaps best illustrates just how difficult it is to achieve.
As well as Federer and Nadal, there's Martina Navratilova, Billie Jean King, Chris Evert, Pete Sampras, Jimmy Connnors, Bjorn Borg and Jimmy Connors to name but a few. And in actual fact none of those male players won all four majors across the entirety of their careers.
The 'Serena Slam' was the epithet Williams came up with when she won the 2003 Australian Open, and thus held all four grand slams but not in a calendar year.
Williams completed the 'Serena Slam' again this year by winning Wimbledon in July, having won last year's US Open. So winning this year's US Open would complete the calendar Grand Slam, not the Serena Slam.
The 'Serena Slam' is not unique to the American; winning four majors in a row has been accomplished by six other players - Budge, Connolly, Laver (twice), Court, Graf (twice) and Navratilova.
Three players have won six majors in a row - Budge between 1937 and 1938. Connolly (1952-1953) and Court (1969-1971).
If Serena wins Saturday's final, she will have won five slams in a row.
Golf is the obvious one here, given that like tennis it is an individual sport with four major events each year.
Never in golf's modern history has a player won all four majors in the same year.
In 1930 Bobby Jones won all the major events played that year, but the events were different to what we have today, with the U.S. Amateur and the British Amateur being latterly replaced by the Masters and the USPGA.
Tiger Woods has come closest to emulating Jones by holding all four majors at the same time, but his were spread across two calendar years (2000-2001).
In Snooker, just three men have won all three Triple Crown events in a single season - Steve Davis (1987/88), Stephen Hendry (1989/1990 and 1995/1996) and Mark Williams (2002/03).
Athletics has no real equivalent because athletes are restricted in what events they can enter according to their nationality. For a British athlete the holy grail is to hold Olympic, world, European and Commonwealth titles simultaneously, and this has been achieved by just five people (Sally Gunnell, Daley Thompson, Linford Christie, Jonathan Edwards and Greg Rutherford).
Well firstly, a win at the next major (the 2016 Australian Open) would tie Williams with Budge, Connolly and Court on six consecutive grand slams won.
More importantly for Williams winning on Sunday would take her to 22 slams won in total, putting her level with Graf.
She would look to overtake the German's total in Melbourne and then close in on Court's all time high of 24.
Since many of Court's slams were won before tennis's open era began, most would consider Williams the greatest player ever is she were to get past Graf.
Somehow though you can't imagine Serena will rest until she has beaten Court's record, or knowing her, obliterated it.
With 2016 being an Olympic year, Williams would also have the opportunity to win the Golden Grand Slam and emulate Graf's landmark achievement of 1988.
But let's not get ahead of ourselves...
The Williams Family show took centre stage at the U.S. Open with Serena and Venus both cruising to straight set wins on Sunday to set up a monster quarter-final between the tennis siblings.