The destiny of the world number one ranking now lays in the hands of defending champion Naomi Osaka after her two main contenders for the top spot, Ashleigh Barty and Karolina Pliskova, crashed out of the US Open overnight.
Barty, bidding to reach her third Grand Slam quarter-final this year, went down to China’s Wang Qiang in straight sets, committing nearly 40 unforced errors and being unable to break the 18th seed’s serve on nine attempts.
This meant that, for the third consecutive major, she has missed out on a meeting against Serena Williams, who advanced to her 11th straight quarter-final at Flushing Meadows after defeating Croatia’s Petra Martic in straight sets.
The pair had also been slated to meet in the fourth round of the French Open and the quarter-finals at Wimbledon, but it did not eventuate with Williams losing to Sofia Kenin at Roland Garros and Barty being beaten by Alison Riske at the All England Club.
The American will surely now be the favourite from the bottom half of the draw to advance to the final, where she will again have the chance to equal Margaret Court’s record of 24 Grand Slam singles titles.
As for Barty, she will end 2019 as the only female player to have reached the second week at every major this year, reaching the quarter-finals of the Australian Open, winning the French Open and reaching the fourth round at both Wimbledon and the US Open.
The Australian, however, remains alive in the doubles, where she is partnered with fellow former world number one Victoria Azarenka. The pair will play the young American pairing of Cori Gauff and Caty McNally in the fourth round of that division this morning (AEST).
If they win, they could face top seeds Timea Babos and Kristina Mladenovic in the quarter-finals.
Third seed Pliskova followed Barty out of the tournament, losing in three sets to Great Britain’s Johanna Konta, who by winning in three sets completed her set of having reached the quarter-finals or better at each major.
It is the first time since 2015 that the Czech has failed to reach the last eight at Flushing Meadows, where she reached her only major final in 2016 which she lost to Angelique Kerber.
For Konta, it is the third consecutive major in which she has advanced to the last eight, after reaching the semi-finals at the French Open and the quarters at Wimbledon earlier this year.
In what will be her first US Open quarter-final, the Sydney-born Brit will face fifth seed Elina Svitolina, who put an end to Madison Keys’ tournament with a 7-5, 6-4 victory, during which she did not face a single breakpoint.
Barty and Pliskova’s departures mean that all that Naomi Osaka needs to do is to successfully defend her title to retain top spot when the rankings are updated following the conclusion of the tournament.
Otherwise, even a loss in the final will see her lose the world number one ranking to Barty, who earned no fresh rankings points from her run (she also lost in the fourth round last year).
In the men’s draw, Alex de Minaur’s hopes of a first Grand Slam quarter-final were extinguished by a resurgent Grigor Dimitrov, who broke once in each set to win in straight sets and set up a last-eight blockbuster against his idol, Roger Federer.
The Swiss Maestro, who is aiming for a 21st major and sixth US Open title, was ruthless as he thrashed Belgian David Goffin for the loss of just four games, including a bagel in the third set.
Fifth seed Daniil Medvedev broke new ground, reaching his first major quarter-final after defeating German qualifier Dominik Kopfer in straight sets.
He will next face Stan Wawrinka, who advanced when defending champion Novak Djokovic retired due to a shoulder injury at two sets down and trailing 2-1 in the third.
It marks the Lausanne native’s fifth win over a world number one in nine attempts at a major, having also defeated Djokovic twice in a major final as well as defeating Rafael Nadal in the final of the 2014 Australian Open and Andy Murray in the semi-finals of the 2017 French Open.
For Djokovic, it is the second time that he has had to abort his title defence at a major after retiring to Andy Roddick in the quarter-finals of the 2009 Australian Open; on that occasion, he cited heat stress as the reason for his retirement.
It is also the fourth time overall the Serb has retired from a Grand Slam match after doing so against Rafael Nadal in his first major quarter-final at the 2006 French Open, and again to Nadal in the semi-finals of Wimbledon in 2007.
This now increases the likelihood of a first-ever Federer versus Nadal championship showdown in New York, with Stan Wawrinka (in Federer’s half) and Marin Cilic and Alexander Zverev (both in Nadal’s half) the likely only obstacles to this occurring.
The final pieces to the quarter-final puzzle will be solved this morning, with women’s defending champion Naomi Osaka set to face a player who’s beaten her twice this year, and second seed Rafael Nadal to face off against ex-US Open champion Marin Cilic.
Osaka will open play on Arthur Ashe Stadium, after which Alexander Zverev will bid to reach his first Grand Slam quarter-final outside of Roland Garros when he comes up against Argentina’s Diego Schwartzman, a quarter-finalist here in 2017.
The clash between Cilic and Nadal headlines the evening session, before Bianca Andreescu, the third round victor of Caroline Wozniacki, takes on American Taylor Townsend with both players bidding to reach their first major quarter-final.
Former Wimbledon semi-finalist Julia Goerges will start a favourite to reach her first US Open quarter-final when she comes up against Donna Vekic on Louis Armstrong Stadium, while French showman Gael Monfils will feature in the late afternoon.
The doubles match featuring Ashleigh Barty and Victoria Azarenka versus Cori Gauff and Caty McNally is second on the Grandstand.
Matches to watch on Day 9
Arthur Ashe Stadium
Day session – play starts at 12:00pm (2:00am AEST)
 Naomi Osaka (JPN) versus  Belinda Bencic (SUI).
 Alexander Zverev (GER) versus  Diego Schwartzman (ARG).
Night session – play starts at 7:00pm (9:00am AEST)
 Marin Cilic (CRO) versus  Rafael Nadal (ESP).
Taylor Townsend (USA) versus  Bianca Andreescu (CAN).
Louis Armstrong Stadium
Play starts at 11:00am (1:00am AEST)
 Donna Vekic (CRO) versus  Julia Goerges (GER).
Not before 12:30pm (2:30am AEST)
Andrey Rublev (RUS) versus  Matteo Berrettini (ITA).
Kristie Ahn (USA) vs  Elise Mertens (BEL).
Not before 5:00pm (7:00am AEST)
 Gael versus Monfils (FRA) vs Pablo Andujar (ESP).
Second match – Cori Gauff (USA)/Caty McNally (USA) vs  Victoria Azarenka (BLR)/Ashleigh Barty (AUS).