American star Tommy Paul dismantles plucky qualifier Gabriel Diallo to set up Jannik Sinner showdown in US Open last 16

There was a telling moment at the end of the first set, as young Canadian Gabriel Diallo danced back to his chair having just won the tie-break to take an unexpected lead over American 14th seed Tommy Paul.

As he crossed paths with his opponent, Diallo pumped his fists and roared up at his coach in the stands. Paul, looking up at him, smirked back as if to say: 'Just wait'. 

Fast forward 39 minutes, and parity had quickly been restored. Paul jogged back to his chair, and it was his turn to celebrate. Diallo was back to earth with a bump, a reminder that Grand Slams can humble you very quickly.

In the end, Paul did all he needed to in front of a raucous Louis Armstrong Stadium, beating his 22-year-old opponent in four sets to book a place in the last 16 and secure his $325,000 in prize money.

The American started slowly against an opponent he'd never played before, taking well over an hour to feel his way through, before eventually getting to work.

Tommy Paul roars with delight after winning the second set and leveling the match

Paul proved a match too far for young Canadian qualifier Gabriel Diallo on Saturday

It has been quite a ride for Diallo, the qualifier with a Guinean father and Ukrainian mother, who honed his trade in Kentucky on the American college scene, and was a match point away from not even making it to the main draw of his first US Open.

At six-foot-eight, and with a booming serve, there aren't many players in the draw more formidable to stand across the net from, but his inexperience showed under the bright lights of the show courts in the biggest match of his career so far. 

He came through two brutal matches earlier in the week, first beating Jaume Munar and then stunning 24th-seed Frenchman Arthur Fils in front of a packed Court 17.

In the first week of the tournament, he has spent three hours longer on court than Paul, and perhaps that showed as the third round progressed here on Saturday.

The first set was a slog, with neither player seemingly able to hold serve. Paul broke to take a 4-3 lead, before Diallo broke back for 4-4. Paul then broke again, and Diallo did the same. From there, another Paul break, and then another for Diallo. Six in a row to take the first set to a tie-break.

Serena Williams (left) was in the house to watch Paul's win over Diallo on Louis Armstrong

Diallo had already spent seven hours on court across his first two matches at the US Open

After that, and Diallo's ecstatic celebration, the tide began to turn in the American's favor. Apart from a small section of Diallo fans at the back of the lower tier, this was a partisan home crowd, all vying for the man who grew up a mere 100 miles away in New Jersey.

In the second set, the errors began to creed in for Diallo, who went 3-0 down quickly and then eventually succumbed 6-3. It was worse in the third set, Paul running out 6-1 winner to really assert his dominance on the match. 

Credit to Diallo, he again rallied and kept pace with his rival, taking him to a tie-break in the fourth to tease the possibility of a deciding set. How the fans enjoyed that.

Tougher tests await for Paul, who will now face world No 1 Sinner in the last 16

Alas, Paul stepped up his game and won it 7-3, seeing out the match on Diallo's serve with a wonderful forehand winner down the line. 

Now, for the American, it is time to look forward, and as far as Grand Slam last 16s go, the draw doesn't get much clearer than this. No Djokovic, no Alcaraz, and 10 of the top 20 seeds already on flights home from New York.

The problem for Paul? It's Jannik Sinner - the best player in the world - up next. 

By getting to the fourth round he's already matched his best-ever finish at the US Open, but the real hard work starts now.