In retrospect, it's clear we have Virginie Razzano to thank for all of this.
She was the No. 111-ranked French player who knocked Serena Williams out of the 2012 French Open in the very first round -- something that hadn't happened to her in 46 previous Grand Slams.
Afterward, Williams reached out to a French coach, Patrick Mouratoglou, and asked him what he thought of the match. Mouratoglou, correctly, said Williams looked emotionally distressed and physically out of sync. They started working together and Serena went on a 19-match winning streak that included titles at Wimbledon and the US Open. Her record the rest of the season under Mouratoglou was 36-1.
In the 13 majors they've been a team, Williams has won eight.
"For me, he's the biggest reason she's been better," said Brad Gilbert, an ESPN analyst who coached Andre Agassi, Andy Roddick and Andy Murray. "It's not even close [to what it was] in terms of coaching and preparation. Serena used to be all about being athletic.
"Now, Patrick studies tape and looks at the things they can exploit, strengths and weaknesses. I feel like she's 10 times more prepared, with strategy and game plans, match in and match out."
For Paul Annacone, now a Tennis Channel analyst, the difference has been Williams' ability to manage her sometimes wild, emotional swings on the court.
"She has always had a lot of passion," Annacone said. "Now she is finding ways to manage those emotions. Look how many times she's been able to win matches when she's playing average tennis. It's that mental edge that puts her over the top."
ESPN analyst Mary Joe Fernandez, who was one of Steffi Graf's victims in 1988 -- she lost 6-2, 6-2 in the fourth round at Wimbledon -- marveled at Williams' ability to repeatedly escape imminent danger.
"In Paris especially," Fernandez said. "She lost the first set four times in five matches.
"What's going to happen when she finds the next gear and truly dominates? I'm still waiting for her to play her best tennis."
Playing some of her best tennis this year, Williams won five of the first six games in the recent Toronto semifinals against Swiss teenager Belinda Bencic.
But then, curiously, she began to disappear. The forceful Bencic won six straight games -- and seven of eight -- in a span of the second and third sets and stunned Serena 2-6, 7-5, 6-4. Bencic, 18, was the youngest player to beat Williams in a completed match since a 17-year-old Maria Sharapova won their finals match at the 2004 WTA Finals.
This is precisely the scenario by which Serena, seeking here 22nd Grand Slam singles title, could lose in New York.
"She has to come up against an opponent with no fear," Cahill said. "Secondly, she has to have a bad day at the office. Those two things happened in Canada.
"So far she's proved herself the best player when it comes down to walking on the court and believing she can make it happen."
The leading question? How will Williams handle the pressure of chasing this almost unimaginable feat, that potentially suffocating weight of expectation?
"Serena's biggest competition is Serena," said Annacone. "I think she's shown she's head and shoulders above everyone else. She won in Paris playing average for her. She won Wimbledon playing average most of tournament.
"Now, if she's comfortable, if she can manage her own nerves, she will win the Open. If she doesn't manage those nerves, she's vulnerable to anyone in the top 20."
As ESPN analyst Pam Shriver points out that, although Williams has won all 21 major outings this year, she has been consistently inconsistent within those matches.
"I think the body of work suggests she wishes she had fewer ups and downs," Shriver said. "I actually think she'll be on guard for those early upsets. I'm trying to think of a player equipped to do it. [Victoria] Azarenka probably has the best attitude walking out there."
Indeed, Azarenka is one of two players to have lost twice to Williams in Grand Slams this year -- but she pushed Williams to three sets both times, winning the first set 6-3 before falling at Roland Garros and Wimbledon.
The 20th-ranked Azarenka, however has lost 17 of 20 career matches against Williams. No. 2 Simona Halep and No. 3 Maria Sharapova are a combined 3-24, not including Halep's walkover earlier this year at Indian Wells. Even No. 9 Garbine Muguruza, who has played two solid matches against Williams this year but lost them both, has won only one of their four career matches.
For Gilbert, it's not merely a matter of who can challenge Williams in New York. He wonders about the foreseeable future.
"I don't see the person who's going to beat her consistently," Gilbert said. "It's probably somebody who's 16 or 17 right now. It's really a matter of how long Serena wants to keep going. We may never see another one like her.
"At this point in her career, she's just remarkable in being able to find that resolve in the big moments. Things slow down for the Hall of Fame greats when the chips are the highest. Like [Michael] Jordan, and Federer, she has a clear picture of what she needs to do."
After losing the Cincinnati final, Halep had some extraordinary, heartfelt words for her opponent at the trophy presentation.
"I have to say congratulations to the champion, Serena, and wish you good luck at the US Open," Halep said. "I know that you can win all four -- so good luck, and be strong."
Williams, too, knows she can win all four. Now she just has to do it.