Roger Federer’s shocking loss to John Millman at the 2018 U. S. Open could cost him more than just a berth in the quarterfinals.

The stunning 6-3, 5-7, 6-7, 6-7 fourth-round collapse denies Federer another chance to defeat Novak Djokovic, perhaps the biggest obstacle to the 20-time Grand Slam champion’s claim to greatest of all time.

As Federer cruised through the draw, a 47th showdown with Djokovic appeared inevitable. After Djokovic beat Joao Sousa 6-3, 6-4, 6-3, a seventh clash with Federer at Flushing Meadows seemed certain.

Yet, after one of the most perplexing performances in Federer’s career, Djokovic gets an exhausted, even if exhilarated, Millman in the quarters.

After the match, Federer told reporters that the humidity got to him. “John was able to deal with it better. He maybe comes from one of the most humid places on earth, Brisbane,” said Federer in his post-match press conference. “I knew I was in for a tough one. Maybe when you feel like that, as well, you start missing chances, and I had those. That was disappointing. But at some point, also, I was just happy that the match was over.”

The loss is more than merely disappointing. Any missed opportunity to best Djokovic costs Federer a chance to solidify his legacy. He desperately needs more wins over Djokovic. 

Although Rafael Nadal has a lopsided head-to-head against Federer, it’s Djokovic who poses the biggest threat to any claims of greatest ever.

Djokovic has seven Grand Slam titles fewer than Federer. But Grand Slam totals aren’t the reason that Djokovic is the problem and the solution to Federer’s status among the greats.

Nadal holds a 23-15 advantage over Federer. Fed fans write that lopsided head-to-head off as a “clay court thing.”  Having a losing record to one guy in his era, the King of Clay, tinges but doesn’t tarnish Federer’s legacy. But what about losing records to two contemporaries? 

Djokovic has a 24-22 advantage over Federer. However, Djokovic is the only member of the Big Four with a winning record against the other three. With losing records against two of the Big Four, how can Federer claim greatest of all time?

Can you be the greatest ever if you weren’t even the greatest among these legends in your era?

Therefore, Federer needs wins against Djokovic. Overtaking Nadal in the head-to-head is unlikely. Djokovic, however, is within reach. If Federer can at least even the score with Djokovic, it erases a sticking point in how his career will be judged.

Federer’s legacy is tethered to his Big Four deficits. Despite Andy Murray’s recent struggles with injuries, he was once part of the Big Four. That era may be gone, but it won’t be forgotten. Each man’s legacy is tied to how they performed against the others.

Before Djokovic, Nadal and Murray emerged, Federer’s greatness was measured against Pete Sampras, whose 14 Grand Slam titles used to be an ATP record. Now even Nadal (winner of 17 Slams) looks at that record through a rear-view window. Djokovic could win his 14th this weekend.

Some see Federer’s 20 Slam titles partially due to longevity more than superiority. In 2015, when Djokovic won three of the four Grand Slams, ESPN UK writer Simon Barnes declared Djokovic the greatest.

“Djokovic is probably the finest player who has ever played. That is to say, if the players from former eras were to take him on with the weapons they possessed at their peak, they would all lose,” wrote Barnes. “Djokovic rose to the top in an era dominated by the rivalry between Rafael Nadal and Roger Federer. He has moved on to eclipse them both: his ascent was their decline.”

Of course, Nadal and Federer regained the No. 1 ranking, but that was when Djokovic was himself in decline.

Djokovic seems to have rediscovered his dominating ways, which means Federer’s chances will be fewer, tougher.

Federer hasn’t won at the U. S. Open since 2008. He has that one French Open title and eight Wimbledon titles. Could being the King of Grass be used against him in the same why clay dominance dogs Rafa?

Speaking of Nadal, when asked if Federer were the greatest, the Spaniard gave his Swiss nemesis a nod based on stats. But when it came to measuring the men as competitors, Nadal seemed to think Djokovic was the tougher opponent.

So, you have someone who dominates Federer, giving the nod to Djokovic.

Djokovic missed last year’s U. S. Open due to an elbow injury that sidelined him for six months. 

Last year, Dominic Thiem told Sports 360’s Reem Abulleil that he expected Djokovic to make a triumphant return as Federer and Nadal did after injury setbacks.

 “I think we can expect the same (from Djokovic) because he is I think in the same league like Roger and Rafa, also one of the greatest players of all-time,” Thiem said. “I think he’s even probably more dangerous and better if his injury is fine and if he had some time to recharge everything.”

Last month, Djokovic defeated Federer 6-4, 6-4 at the Western and Southern Open in Cincinnati. With that victory, Djokovic became the first men's tennis player to win all nine Masters 1,000  tournaments, another record he has that Federer doesn’t.

If Federer can’t right the ship against Djokovic, Nadal might be the next best hope. Nadal and Djokovic could meet in the finals.

Djokovic has a 27-25 advantage over Nadal. If the Spaniard can erase that deficit, Djokovic would no longer stand alone as the only member of the Big Four with a winning record against the others. However, that would give Nadal that distinction and that means…

Yeah, Federer needs more wins against Djokovic.