FEBRUARY 2013 was a big month for the ratings industry. While the UK lost its top AAA credit rating for the first time since 1978, in the world of tennis another bankable asset was resuming her place at the summit of the rankings. It is now 183 weeks and counting since Serena Williams was last regarded as anything other than the top female tennis player in the world.

In the three-and-a-half years since, the 34-year-old from Michigan via Florida has comprehensively rewritten the history books, amassing sums equivalent to the GDP of a small European country. Serena goes into her home slam at Flushing Meadows this fortnight chasing a seventh US Open title, the most of any woman in the Open era. Victory would present her with a 23rd Grand Slam title in all, enough to move her clear of Steffi Graf in the list of major champions in the Open Era. Margaret Court's all time best of 24 is also in her sights, even though 11 of them arrived after major championships were opened up to all comers.

But for the first time in a long while, she does so at a time when that supremacy at the top of world tennis is under serious threat. Germany's Angelique Kerber stands just 190 points back after a stellar year which has seen her win the Australian Open and fall just short at Wimbledon, with no little help - co-incidentally or not - to a pep talk from her mentor Steffi Graf, whose record of 186 consecutive weeks at No 1 is also in Serena's crosshairs.

There are other threats too: Garbine Muguruza may be out of form but was an impressive winner of the French Open title, while new Olympic champion Monica Puig of Puerto Rico is another exciting new star in the firmament. Serena complained of a shoulder problem and obliterated a racket or two when going down to Elina Svitolina of Ukraine in the third round in Rio and Flushing Meadows meltdowns, such as her semi-final setback against Italy's Roberta Vinci 12 months back, are not entirely unheard of.

"I was with Serena in Rio," said Mary Joe Fernandez, part of the USA Olympic coaching team who is also a pundit for ESPN. "She definitely was not 100 per cent physically with her shoulder so I think she made the right choice by not playing in Cincinnati. It was tough for her because the No 1 ranking is important to her, and she'd like to not just tie Steffi Graf for No 1 at consecutive weeks, but she'd like to break it.

"It's great that it's in her hands. If she wins the US Open, she'll keep the No 1 ranking. She's only played seven tournaments this season, which is not that much. So it was inevitable that players were going to catch up, and Kerber has had such an amazing year by winning the Australian and reaching the finals at Wimbledon. Muguruza is pretty close, too. It makes it exciting."

Chrissie Evert, another ESPN pundit who also won six US Open titles, said: "For Serena, being No 2 in the world doesn't sit well with her. Hopefully Kerber is not burned out, because she has every right to be after this year she's had. But I mean, when I was No 1, there's no way I wanted to lose it. It's a pride thing. It could be an ego thing, too."

While Serena faces Ekaterina Makarova first up - the Russian has a Grand Slam win against the younger Williams sister on her CV from the 2012 Australian Open - another intriguing name coming into the event is Johanna Konta. The Briton is enjoying her finest season and may fancy her chances of a long run if she can dispose of home favourite Bethanie Mattek-Sands first up.

"Konta has been impressive," said Fernandez. "The last 12 months, what a jump. She had to qualify for the US Open last year, was ranked outside the top 110. She has improved in so many categories starting with her serve. I think she has the third most aces for the season. The backhand is very good; that's her weapon. The forehand used to be a weakness but she is the most improved by far in the last 12 months. Can she make a deep run? Definitely."