Following the clay and grass stints of the season, the US Open is the culmination of an exhausting seven-week North American hardcourt swing. Most top players arrive fatigued at the US Open which increases the probability of a lower-ranked player to spring a surprising result against a higher-ranked one.
This explains why the US Open has produced the most number of different winners in the last two decades, compared to the four other Grand Slams, including the most number of first-time Slam winners.
Let us have a flashback of the 5 big upsets in the last decade at the US Open:
#5 2013 Fourth Round: Robredo beats Federer 7-6(3), 6-3, 6-4
Spaniard Tommy Robredo was winless in his 10 previous meetings with Roger Federer, going into his 2013 fourth-round match against the 5-time champion.
On a rain-affected day at the US Open, Federer's match was shifted to the second show court to clear a backlog of matches. The Swiss had been suffering from poor form and a bad back throughout the season but hinted signs of a turnaround by winning his first three matches at Flushing Meadows.
Robredo twice led by a break in the first set only for Federer to force a tie-break where the Swiss took only three points to concede a one-set lead.
In the second set, Federer lost serve twice and dropped three games in a row to fall two sets behind. Hopes of a comeback dissipated in the third set when Federer dropped his serve to love in the seventh game and squandered six break points.
Robredo served out the match to land one of the biggest wins of his career as Federer failed to reach the quarter-finals at the Flushing Meadows for the first time in 10 years.
#4 2014 Semifinal: Nishikori beats Djokovic 6-4, 1-6, 7-6(4), 6-3
Following consecutive five-setters against Milos Raonic and Stanislas Wawrinka in each of his previous two rounds, Kei Nishikori arrived at his first Grand Slam semi-final as the overwhelming underdog against top seed Novak Djokovic.
Displaying little stage fright, the Japanese allayed any concerns of exhaustion when he took the first set for the loss of four games. Djokovic responded with greater firepower in the second to level the match at a set apiece.
In a competitive third set, Nishikori survived seven deuces to hold serve for 2-1 but failed to serve out the set at 5-3 as Djokovic broke back. To his credit though, Nishikori regained control of the match by playing a superior tie-break to take a two-sets-to-one lead.
Riding on his momentum, the Japanese broke Djokovic in the first game of the fourth set and the single break of serve would suffice as Nishikori served out the biggest win of his career to reach his maiden Grand Slam final.
In the process, Djokovic's bid to reach five consecutive US Open finals came to an end.
#3 2018 Fourth Round: Millman beats Federer 3-6, 7-5, 7-6(7), 7-6(3)
On a sultry New York evening, Federer took a one-set lead but saw his level plummet in the second set as his first serve spectacularly started misfiring. The Swiss managed to survive seven break points in the fourth game and broke Millman in the next game to take a 3-2 lead.
Federer arrived at two set points (40-15) when serving for the second set at 5-4 but remarkably faltered as the Australian won four points in a row to break back. Serving to stay in the set at 5-6, Federer once again lost four points in a row from 40-15 as Millman leveled the match at a set apiece.
Willed on by a passionate Arthur Ashe crowd, Federer's level improved in the third set and the Swiss had a set point on the Millman serve only for the Aussie to fire a second serve ace and go on to take a two-sets-to-one lead.
Federer broke for 4-2 in the fourth set but another poor service game from the Swiss helped Millman back into the set who went on to win the ensuing tie-break to seal one of the biggest wins of his career.
Unable to control the match from the baseline, Federer resorted to 35 drop shots but the Australian punished the Swiss in most of them en route to a monumental upset.
#2 2015 Third Round: Fognini beats Nadal 3-6, 4-6, 6-4, 6-3, 6-4
Rafael Nadal made a bright start to his face-off against Fabio Fognini in the third round of the 2015 US Open, where he led by two sets and a break in the third. But the Italian seemed to momentarily stall Nadal's victory bid by taking five of the last six games to take the third set.
Trailing two-sets-to-one, Fognini faced an uphill task against a player who had never lost a Grand Slam match on 152 previous occasions after taking a two-set lead.
The Italian failed to rein down his unforced errors but produced more winners to take the match to a decider where Fognini thrice led by a break, only for Nadal to recover the break back in the very next game each time.
Fognini's heavy hitting forced a fourth break of the Nadal serve in the fifth set. The Spaniard was powerless to retrieve the break and the Italian held serve to record one of the finest wins of his career to reach his first fourth round at the US Open.
Defeat to the Italian ended Nadal's record of winning a Grand Slam in ten consecutive years.
#1 2009 Final: Del Potro beats Federer 3-6, 7-6(5), 4-6, 7-4(4), 6-2
In his sixth US Open final in as many years, five-time defending champion Roger Federer was an overwhelming favourite against young Argentinian Juan Martin Del Potro.
Federer lived up to his top billing by romping to the first set for the loss of just three games. Del Potro looked overawed by the occasion as Federer seemingly took control of the match, as he served for the second set and arrived at 30-0.
However, a contentious video referral at 30-30 perturbed the serene Swiss who lost four consecutive points to drop serve. Del Potro capitalised on Federer's lapse to take the second set and level the match at a set apiece.
The Argentinian broke Federer in the third set and led 4-3, only for the defending champion to respond by reeling three games on the trot to take a two-sets-to-one lead.
With the momentum now firmly in his favour, the Swiss looked to be cruising to his sixth title at 5-4, 30-30 on Del Potro's serve only for the Argentinian to dash the Swiss maestro's hopes by forcing a decider.
Federer couldn't withstand the barrage of heavy forehand artillery from the Argentinian in the final set as Del Potro ended the five-time champion's reign to celebrate a maiden Slam title. It also enabled him to become the youngest US Open champion since Lleyton Hewitt's win in 2001.