NEW YORK -- In February 2013, during a four-week span that saw President Obama inaugurated for his second term, Michael Jordan turn 50 and the Pentagon lift the official ban on women in direct combat roles, Serena Williams returned to the top of the world rankings in tennis. For the sixth time.Other than the fact that Williams, at age 31, became the oldest woman to hold the top spot since the computer rankings were introduced in 1975, surpassing Chris Evert, it wasnt huge news.Eleven years earlier, at age 20, Serena had attained the No. 1 ranking for the first time and spent a little over a year there. She returned to the top again in September 2008 after more than a five-year gap, the longest span between stays at No. 1 in WTA history.At no time since then would a change at the top have been quite so dramatic. But it will be the next time it occurs.At the end of the US Open, Williams, who plays Yaroslava Shvedova in a fourth-round match Monday, will either be tied with Steffi Graf for the longest stay at the top at 186 weeks, or she might lose the ranking altogether.While Williams held a 190-point advantage over No. 2 Angelique Kerber heading into the US Open, Kerber, the No. 2 seed here, now holds a 650-point lead after defeating No. 14 Petra Kvitova 6-3, 7-5 on Sunday night to advance to the quarterfinals against Roberta Vinci.To remain No. 1, Williams will need to reach the Open final. But if Kerber also reaches the final, Williams will need to beat her to keep the top ranking.Agnieszka Radwanska also has a chance at No. 1 but needs to win the title.Kerber has said she has put the possibility of No. 1 out of her mind.Actually its easy, Kerber said after her victory Sunday night, her 51st match win in 2016. If one day it happens, it will happen and will be the best day. But its still a long, long way to go. Im now in the next round and thats what counts right now.With the gap between Serena and the rest of the field seemingly insurmountable over the past three years, and Williams still capable of dominating tennis when she is in top form, losing the top ranking would no doubt catch a lot of people off guard.What it will not do, however, is change many opinions within the game.I think Serena for most of the people, is going to be No. 1 anyway, said Lesia Tsurenko, who lost to Vinci on Sunday in a fourth-round match. She is the greatest player ever. But its nice whats going on now in tennis that anyone can beat anyone. Thats probably good and I think this is also good for people who watch tennis just to see some new faces.Johanna Konta, who lost her fourth-round match Sunday to Anastasija Sevastova, said any of the No. 1 scenarios would be a wonderful achievement.[But] Serena maintaining her No. 1 position, I dont think says anything less of the depth of womens tennis, Konta said when that was suggested. I think the level that the girls are playing on a consistent basis is pretty impressive, and its only going to get better and better.But lets not dethrone Serena quite so fast. While the soon-to-be 35-year-old has won just one major this year, that Wimbledon title gave her 22 major championships, tying her with Steffi Graf for an Open-era record she will surely hold alone if not next week then at some point next year.For Vinci, who made Serena look particularly vulnerable with her historic upset in last years US Open semifinals, thus derailing Williams pursuit of the calendar Grand Slam, she was in no hurry to unseat the world No. 1.Well, its interesting, Vinci said. Kerber may be No. 1, but its tough. Its tough to be No. 1. Serena is still here. I saw her yesterday [defeat Johanna Larsson, 6-2, 6-1]. Shes on fire. We will see. Its not easy to be No. 1. The points and the pressure, well see. But Serena, I saw her. She played so well.