Exactly 27 years ago, Steffi Graf achieved one of tennis’ greatest feats. On Sept. 10, 1988, Graf beat Argentina’s Gabriela Sabatini in the final of the U.S. Open to become just the third woman ever to win a calendar-year Grand Slam -- a clean sweep of tennis’ four Major titles. It is a testament to the difficulty of the achievement that no player has matched it since. Potentially until Saturday, that is.

Providing she gets through her rain-delayed semifinal with Roberta Vinci on Friday, Serena Williams, already the winner of four consecutive Grand Slams for the second time in her career, will play for the right to join Graf, Margaret Court and Maureen Connolly as the only female players to have won it all in a single season.

But it is not just Graf’s greatest year that Williams stands set to match. Further emphasizing what a historic day Saturday may prove to be at Flushing Meadows, were Williams to hold the trophy aloft, she would also join Graf on 22 Grand Slam titles -- the most of any player in tennis’ Open era.

In a 17-year career that began at the age of just 13, Graf set records that many thought would never be rivaled. But as soon as one great champion faded from view, another was already stepping into the void.

It was at the French Open in 1999 that Graf won her 22nd and final Grand Slam title, putting her four ahead of legends Chris Evert and Martina Navratilova and just two short of Margaret Court on the all-time list. A month later she would reach the Wimbledon final losing to Lindsay Davenport. And a month after that she announced her retirement, turning her focus instead to her burgeoning romance with another tennis great, Andre Agassi, becoming a mother and her charity work.

At the time Williams was a 17-year-old, and in the shadow of sister Venus, who, as a 19-year-old reached the quarterfinals of Wimbledon, where she lost to Graf. Yet in the first Grand Slam since Graf’s retirement, it was Serena who stunningly triumphed to rapidly usher in a new era for the sport and began her ascent to what may go onto become the greatest career in the history of women’s tennis.

Before that, the paths of Serena and Steffi crossed over twice, with Graf claiming victory in three sets in Sydney at the start of 1999, before Williams turned the tables in Indian Wells. While her memories are patchy, in a week where she put her name alongside the Graf, Williams discussed what made the German so special.

“She was Steffi Graf,” she said after beating Venus in the quarterfinals. “I think that's what made her really tough. You know, when you're young and going against Steffi Graf, I mean, that, I think, pretty much sums it up.

“It's been a really long time since I have played her, but I just do remember her having an unbelievable forehand. I think her backhand was amazing, too, because she had that really good slice. She was very athletic and very fast. She did a lot of things really well.”

Indeed, it was her athleticism that set her apart when she burst onto the scene. “She gets to balls no one else can,” Martina Navratilova said after she had been beaten by Graf in the 1988 Wimbledon final in a match that represented a passing of the torch in women’s tennis.

More than just her incredible speed, the potency of Graf’s forehand began the move toward the dominance of power hitting from the back of the court and away from the serve-and-volleying style exhibited most famously by Navratilova. Graf would go on to have her greatest success at Wimbledon, where her combination of serve and forehand was deadly, and her sliced backhand deceptively effective. But Graf’s greatness came from the fact that she was the best on all surfaces. She won five titles each on the hard courts of the Australian and U.S. Opens, and added six on the clay of Roland Garros.

During her prime the only serious threat to her dominance came from Monica Seles. The Yugoslavian great won eight Grand Slam titles between 1990 and 1993, when her career was derailed by a mentally-ill fan of Graf’s running onto the court and stabbing her in the back during a match in Hamburg.

While Williams is on the verge of matching Graf’s most famous accomplishments, some will remain firmly in place. For one, Graf’s 1988 calendar-year Grand Slam came complete with an Olympic gold in Seoul. And the dominance she enjoyed in her era is demonstrated most obviously by the fact she spent 377 weeks ranked No. 1 in the world, more than any other player in history. Williams currently sits on 257.

The debate about which of the two has had to compete in the toughest era will continue to rage. But certainly the dramatic global expansion of the game from the time Graf’s career was getting going to now is a powerful argument in Williams’ favor. In 1985, the top 100 women came from just 24 countries. That number now stands at 31. The growth of the game in Eastern Europe and Asia in particular means that competition is now coming from far and wide like never before.

Perhaps more than that, just as Graf did, Williams has undoubtedly taken the game to new levels. Serving with a potency previously unimaginable in the women’s game, capable of producing incredible power off both forehand and backhand as well as setting new standards for athleticism, whether she wins the title on Saturday or not, she is surely already the greatest ever. Graf, though, set a phenomenal target.