Serena just doesn't spark the same feeling of inevitability

— -- NEW YORK -- It was late in this US Open semifinal match, and once again, the lopsided nature of Serena Williams' r?sum? versus her opponent's didn't matter. Same as it didn't save Williams in four of her past five Grand Slam losses before this. This time, Williams' tormentor wasn't her Australian Open conqueror Angelique Kerber, or Spanish upstart Garbine Muguruza, who surprised her in this year's French Open final. It wasn't even a wily veteran like Roberta Vinci, who derailed Williams' run at a calendar-year Grand Slam on this same court a year ago with arguably the most monumental upset in tennis history.

Thursday night on the floor of Arthur Ashe Stadium, the new predicament Williams found herself in was a win-or-go-home tiebreaker against 24-year-old Karolina Pliskova, a 10th-seeded but largely unproven Czech. Williams was faced with the unenviable chore of trying to survive against the most prolific ace machine on tour. This while playing with an aching left knee -- a new ailment that only a few insiders knew that she had until after the match, which ended ingloriously for her with a double fault that sealed Pliskova's stunning 6-2, 7-6 (5) win.

Just like that, all the history Williams was chasing -- a tie with Chris Evert for the record of seven US Open titles, a chance to break her tie with Steffi Graf at 22 majors, most in the Open era -- was put off for another day. When Williams wakes up Monday morning, she also officially won't be ranked No. 1 (Kerber will) leaving Williams forever moored in a tie with Graf for most consecutive weeks in the top spot, with 186.

"I'm not talking about No. 1, thank you," Williams said during an occasionally terse news conference in which she allowed that her knee might've contributed to the loss. Williams, though, hotly disputed fatigue from her three-set win over Simona Halep a day earlier was a factor.

For Williams, who turns 35 on Sept. 26, the defeat nonetheless continued a troubling trend. By any measure -- the statistics, the anecdotal evidence, the way she has dominated tennis for two decades now -- Williams is the greatest player in tennis history, bar none. There's no need to confine the praise to only the women's side of the sport.

Including last month's Rio Olympics, where Williams was sent crashing out in the third round by Ukrainian Elina Svitolina, what has been astonishing isn't just who Williams has lost to in the biggest moments the past year -- it's how she has been losing. Even when her body hasn't betrayed her, as it did in the past month, nerves seem to haunt her. Her confidence seems more brittle than you'd expect, given all that she has done. Errors come in bunches. Opponents are rushing in to capitalize.

Williams has now lost four of her past five major tournaments to players that she had a combined career record of 18-2 against before she lost to them.

Players still deeply respect Williams. But other than her win against Kerber for the Wimbledon title, Williams just doesn't project the same self-assurance or strike the same off-the-charts fear into her opponents that she used to. And Muguruza had no problem saying that out loud after her French Open title win, telling reporters that players are seeing that Williams is more "beatable" now.

Of course, Muguruza hasn't personally been able to build on her breakthrough win. She has fallen into the same struggles that Kerber admittedly did after she won her first Slam title. But still other players have performed like they agree with the Spaniard -- playing Williams doesn't spark the same feeling of inevitability it used to. Other players just seem to see a little daylight now against Williams, where before, they saw little to none.

Romania's Halep said as much, too, after very nearly beating Williams in their three-set dogfight on Wednesday night. Afterward, she rued her failure to break Williams in the first game of the final set, saying if she'd done that then, the ending would've been "a different story."

Pliskova played like one of the new believers, too. Williams is generally thought to have the greatest serve in the history of the game, but Pliskova's serve and return games were both better than Serena's on Thursday night.

And the thing is, Pliskova had never even been past the third round of a Grand Slam tournament until this giant-killing 11-match summer winning streak she's on, which also included a win over Serena's sister, Venus, earlier in this tournament. With the victory, Pliskova became only the fourth player to beat both William sisters at the same Slam, joining Martina Hingis, Kim Clijsters and Justine Henin.

"I said I don't believe it -- but actually, I do believe [I won]," Pliskova said. "I always knew I had a chance to beat anyone if I'm playing my game."

For Serena, who is used to doing the dictating, it was a frustrating night. But she refused to go as far as her coach, Patrick Mouratoglou, did and blame this loss on her knee injury. (Mouratoglou said she suffered it against Halep on Wednesday, but Williams said she suffered a few rounds earlier.) The most Williams would allow was being hampered by the knee caused her focus to waver.

"I was making errors that I never make. So many simple, simple shots that I could've easily made," she said.

Williams has been so extraordinary for so long, any wobble in her play seems like a seismic event. But she has now had four such upsets in her past five Slams. She isn't just blowing these matches; her rivals are going out and taking them.

Williams has dug herself out of too many trenches before in her majestic 22-year career to rush to any big conclusions that this is the beginning of her end as the game's greatest player. But it has been happening for a year now.

And it's only natural to wonder.