The highly anticipated semifinal showdown between Roger Federer and Rafael Nadal has one very large obstacle in its way. He stands 6-foot-7, pounds forehands like Zeus throws lightning bolts and has the New York crowd very much on his side.

Juan Martin del Potro staged the comeback of the tournament in the match of the tournament Monday in the Grandstand gloaming, bouncing back from two desultory sets and two match points down to eliminate Dominic Thiem in a five-setter that will go down as one of the most memorable the US Open has ever offered.

With that, he set up a quarterfinal against Federer, whose Arthur Ashe Stadium win over German Philipp Kohlschreiber was largely lost in the long shadow cast by the man nicknamed the "Tower of Tandil," his hometown in Argentina. It will be a rematch of their riveting 2009 US Open final, when del Potro came back from two-sets-to-one down to stun the Swiss, denying him a sixth consecutive men’s singles crown.

Elsewhere around the grounds, Nadal made sure to hold up his end of the potential Roger-Rafa semi, cruising into the quarters, where he was joined by fellow top seed on the women's side, Karolina Pliskova, who also dominated in Arthur Ashe Stadium.

Yet at the end of the day, all the action in the US Open’s showpiece stadium wound up as the sidebar, with the lead story playing out before 8,000 fortunate fans in an arena tucked in the southwest corner of the grounds – the place where Juan Martin del Potro created another chapter in an ever-growing US Open legend.

With that, here’s a look back at Day 8 and a look ahead to Day 9 of the 2017 US Open:

Match of the Tournament

The match was, for all intents and purposes, over. Dominic Thiem was in full flight, the leaping backhands and rolling forehands painting every line and finding every nook and cranny of the court. Juan Martin del Potro looked like he down for the count, ailing with a virus that left him lethargic and uncharacteristically flailing.

The first two sets took just over an hour, with Thiem surrendering just three games. The buzz surrounding the encounter centered more on if del Potro would retire than if he could come back. But then he did, seemingly flipping a switch and stunning an unsuspecting Thiem to take the third set with ease.

Then the match really got going.

Del Potro broke early in the fourth set; Thiem immediately broke back. Then the No. 6 seed broke again, pulling ahead 5-2 and pushing the Argentine to the brink. But with his opponent serving for the match, the 6-foot-7 del Potro stood tall, breaking again to keep the match going. Undaunted, Thiem secured two match points with del Potro serving at 5-6; the 2009 US Open champion erased both with aces. Then he really started rolling. The tiebreak was one-way traffic, Delpo running out to a 6-1 lead and securing the set with a cannonball of a forehand that is sure to be among the hardest the US Open has ever seen hit – a resounding strike that sent the standing-room only crowd into a state of utter delirium.

Then everyone took a deep breath, and the fifth set began. It did not disappoint. Thiem brushed away triple break point at 3-2, bottling a crowd that was ready to erupt. But in the end, he was only delaying what seemed like the inevitable. At 5-4, on del Potro’s second match point, Thiem spun a second serve that was called an ace. Del Potro challenged; the ball was wide. The match was over, Thiem leaving the court and the stage to his opponent, who raised his arms to the sky as the crowd showered him in cheers.

There will be other great matches during the 2017 US Open – perhaps even that ever-elusive Federer-Nadal semifinal. But for sheer drama, played in the intimate Grandstand before boisterous fans, there will be no other match like this one.

play video Nadal vs. Dolgopolov

Match of the Day

Alexandr Dolgopolov is the tour’s most unconventional player, a tennis-playing Cuisinart who mixes slices, spins and driving ground strokes seemingly at random and with an almost pathological aggressiveness. Rafael Nadal, meantime, is the tour’s most disciplined competitor, a man with a precise pre-serve routine and a changeover regimen that is famous for its fastidiousness.

On Monday in Arthur Ashe Stadium, they met in a match that may have lacked for competitiveness but never pure fun. Their rallies alternated between impossibly long and incredibly short, shots flying at odd angles and to locales rarely traversed in a more conventional contest.

The result was fairly one-sided – a 6-2, 6-4, 6-1 victory for the top seed, his 50th win in Flushing and his first trip to the US Open round of 8 since 2013, when he claimed his last Open title – but the journey to get there was a pure tennis fan’s delight, the kind of tennis that makes you appreciate what the world’s finest can do when they perform their craft at the highest level.

Player of the Day

As the seeds have fallen all around her, Karolina Pliskova has soldiered on, one of just two in the Top 8 – along with No. 4 Elina Svitolina – to advance to the fourth round. But it was a struggle. The world No. 1 labored mightily to put away qualifier Nicole Gibbs in the second round and had to stave off a match point against China's Zhang Shuai in the third.

As Week 2 dawned, Pliskova appeared vulnerable to upset, to being the next high seed shown the door. She changed that narrative in short order on Sunday. The 2016 US Open finalist put on a command performance against Jennifer Brady, blitzing the American in 46 minutes to advance to the quarterfinals and re-establish herself as the tournament favorite.

Pliskova quickly took a 5-0 lead in the match, dropped one game and then picked right up where she left off. Cracking winners off both wings, she raced through the match’s last seven games and into the business end of the tournament, 6-1, 6-0. A year ago, Pliskova was the only Top 20 woman never to have reached the second week of a major; now, she is into her fourth in five Slams – looking to take the one extra step and lift her first Grand Slam trophy at fortnight’s end.

September 4, 2017 - Karolina Pliskova celebrates beating Jennifer Brady at the 2017 US Open. (USTA/Garrett Ellwood)
Photo by:  (USTA/Garrett Ellwood)

Upset of the Day

It has been a fine tournament for teenage players. Naomi Osaka bounced the defending champion Angelique Kerber in the opening round, Taylor Fritz advanced to the second round and put a scare into Dominic Thiem, Sofia Kenin did the same in the third round against Maria Sharapova, and Denis Shapovalov set the first week on fire with his electric run to the fourth round.

But of the cohort that began the event, only one is still playing. On Monday, Andrey Rublev upset his second Top 10 seed of the tournament, eliminating No. 9 David Goffin, 7-6, 7-5, 6-3. That comes on the heels of his second-round upset of No. 7 seed and title hopeful Grigor Dimitrov, also in straight sets.

Rublev is hardly an internationally recognized name, but he has a solid pedigree. The 19-year-old Russian, the youngest US Open men’s quarterfinalist since Andy Roddick in 2001, won the French Open boys’ singles title in 2014 and entered this year’s Open at No. 53 in the world. That makes him one of just three teenagers in the Top 100, alongside No. 69 Shapovalov and No. 70 Frances Tiafoe.

Following the 2014 US Open, Rublev was invited to train in Mallorca with 15-time Grand Slam champion Rafael Nadal. The two now face off Wednesday for a spot in the US Open quarterfinals. A third Top 10 upset at this Open and the precocious Russian will be known far and wide.

Number of the Day

4: The number of American women in this year’s US Open quarterfinals, after Madison Keys upset No. 4 Elina Svitolina to close out the night session in Arthur Ashe Stadium. Keys joins CoCo Vandeweghe, Venus Williams and Sloane Stephens in the quarters, setting up the potential for the first all-American semifinal since 1981 (Tracy Austin, Chris Evert, Martina Navratilova, Barbara Potter) if all win their round-of-8 matches.

Four is also the number of qualifiers in recorded US Open history who have advanced to the quarterfinals. Kaia Kanepi became the fourth on Monday when she defeated Daria Kastakina, 6-4, 6-4, in the round of 16. Kanepi joins Barbara Gerken (1981) as the only two qualifying women to reach the round of 8; the men were Gilles Muller in 2008 and Nicolas Escude in 1999.

Quote of Day

“I went from Tiafoe then to the oldies, so that was good. Yeah, back to [guys born in] the 80s.” – Roger Federer, after his fourth-round win over Philipp Kohlschreiber, who defeated 19-year-old Frances Tiafoe in the first round and then three consecutive fellow 30-somethings in the matches that followed

Day 9 Preview

The quarterfinals kick off on Day 9, with eight players fighting for the first four spots in the 2017 US Open semifinals.

Leading off the day session are gritty baseliners Pablo Carreno Busta and Diego Schwartzman, two world-class defenders and returners each looking to advance to a Grand Slam semifinal for the first time. They will be followed by American comeback kid Sloane Stephens, playing in just her fifth event after missing 11 months with a foot injury, taking on Anatasija Sevastova, the 16th seed in search of her maiden major semi.

The day’s marquee match will lead off the night, with Grand Slam champions and fan favorites Venus Williams and Petra Kvitova vying for a spot in the Final Four. And closing the night is Venus’ countryman, Sam Querrey, who will be playing for his second consecutive Grand Slam semifinal when takes on fellow skyscraper Kevin Anderson in a battle of big servers.