In years past, the US Open women's field has often been about Serena Williams....and everybody else.
Among her 23 Grand Slam singles titles, Williams has won the US Open six times, including three straight from 2012-14.
But with Serena out on maternity leave for this year's event, the field is now as wide open as the Northern border on Game of Thrones after the destruction of The Wall.
"There are so many [top players] now that are fighting for the same," said No. 3 seed Garbiñe Muguruza after her 6-0, 6-3 first-round destruction of American Varvara Lepchenko on Monday. "We are all very close, very equal. There are going to be some surprises, as always."
One of those surprises occurred later Monday night when Maria Sharapova, who was granted a wildcard into the Open after serving a 15-month doping ban, stunned No. 2 Simona Halep, 6-4, 4-6, 6-3, in a thrilling match that lasted nearly three hours and was more worthy of a semifinal or a final than a first-round encounter.
Sharapova wore a black Nike dress that featured sequins and lace for her first Grand Slam match since the 2016 Australian Open when she tested positive for the banned heart and blood boosting drug meldonium. With the emotional win, she improved to 7-0 against Halep and 18-0 in night matches at Arthur Ashe Stadium.
"I just thought this was another day, another opportunity, another match, but this was so much more," Sharapova said in an on-court interview after the match. "Behind all these Swarovski crystals and little black dresses, this girl's got a lot of grit. And she's not going anywhere."
Per ESPN, Sharapova became the first women's wildcard to beat a top-2 seed at a major since Kim Clijsters defeated No. 2 Serena Williams at the 2009 US Open. Clijsters went on to win the tournament.
Sharapova now essentially assumes the spot of the No. 2 seed in the bottom half of the draw and could potentially face Muguruza in the semifinals.
A second major surprise occurred Tuesday when 19-year-old Naomi Osaka of Japan routed defending champion and No. 6 seed Angelique Kerber, 6-3, 6-1.
Kerber became just the second US Open women's champion in the Open Era to lose in the first round the following year (Svetlana Kuznetsova, 2005).
Muguruza, meantime, has been installed by bookmakers as the favorite to win back-to-back Grand Slam titles after her Wimbledon triumph earlier this summer, but she isn't ready to acknowledge that.
"In the paper, people might think that," she said. "But you come here and with the conditions and the past, there are certain players that play better and certain players that don't. So far I'm just thinking that I'm happy that I'm in the second round, and that's what I'm going to take."
Muguruza and Sharapova aren't the only women with a shot to win the title.
Entering the tournament, eight women had a mathematical chance to become No. 1 in the world after the Open. Two of them -- Halep and No. 7 Johanna Konta -- lost on Monday, but a half dozen contenders remain: World No.1 Karolina Pliskova, Muguruza, the tour's leader in titles Elina Svitolina, former No.1 Caroline Wozniacki, 2004 US Open champion Svetlana Kuznetsova, and the only woman to make two major finals this year, Venus Williams.
As for Venus, now 37 and a 20-year veteran of the tour, she said she's looking forward to becoming an aunt again, this time to Serena's first child. But she isn't focused on the opportunity to return to world No. 1.
"I just wanna win and if you win you get the rankings," she said after her 6-3, 3-6, 6-2 victory over Viktoria Kuzmova on Monday. "So let's just try to win, that's all I wanna do."
On top of the players in contention for No. 1, others remain legitimate challengers for the Open crown without Serena, too. Madison Keys, 22, is the No. 15 seed and the power-hitting baseliner is looking to lead American tennis into the post-Williams Era.
"I think right now is a really good time for women's tennis," said Keys, who was set to play her opening-round match against Elise Mertens during the night session on Tuesday. "I think there is a lot of depth. I truly believe that, you know, anyone who is playing well could beat anyone the next two weeks. So I think it's really interesting, great time for women's tennis."
Follow Adam Zagoria on Twitter @AdamZagoria for updates throughout the U.S. Open.