Top seed Carlos Alcaraz fended off World No. 13 Tommy Paul 7-6(6), 6-7(0), 6-3 after three hours and 10 minutes in Cincinnati, a match delayed several times due to rain.
On Thursday, Paul met the Spaniard for the fourth time on tour for a place in the Cincinnati quarterfinals.
A tight opener would set the tone for a dramatic third-rounder at the Western & Southern Open hard courts.
On Friday, the winner will face Australian Qualifier Max Purcell, who overcame former world number three Stan Wawrinka 6-4, 6-2 on Center Court.
In his first-round main draw match, Purcell stunned fifth seed Casper Ruud with a solid performance.
High-Level Action on Center Court
A tight start to the opener saw the American player hit first, converting his first break point in the sixth game to earn a 4-2 advantage and holding to extend his lead, saving two breakpoints in the process.
The 26-year-old would serve for the match at 5-3 but rising to the challenge, Alcaraz broke him and held to draw level 5-5.
Under pressure, the 14th seed survived three breakpoint attempts and a 20-point rally, which he lost, displaying great athleticism to turn around the score 6-5.
But then, the Murcia born led the first set to a breathtaking tiebreak which he took 7-6(6) in 62 minutes, winning 77 percent of the first serve points.
Moreover, the 20-year-old achieved the impressive mark of 15-4 in tiebreaks this season.
The players hit back to the court to set off the second set with great intensity, an authentic duel of titans.
The player mentored by Juan Carlos Ferrero is yet to find his A-game after being crowned champion in Wimbledon, playing his second tournament ever since, bidding to build form ahead of the U.S. Open.
The top seed used his vast repertoire to go the distance, finding an edge when he needed the most, but so did Paul at the peak of form.
Overall, the second chapter of a box office Masters 1000 clash featured six breakpoints - three from each side, four in a row towards the end of a monumental second-set battle.
A brutal twelve-game, lasting over 15 minutes, saw Paul fighting off three match points, moving through a tiebreak, which he took by storm 7-0, forcing a third set.
Alcaraz, the only player to win ATP titles on all three surfaces this season, broke to open the decider.
However, the rain made an entrance, adding suspense to a three sets thriller, delaying play several times during the final set.
Ultimately, Alcaraz closed out a contest ruled by small margins to secure his place in the last eight, breaking on his way to the triumph.
He finished with 42 winners to 19 unforced errors, winning 72 percent of the first serve points.
On Tuesday, the 12-time titlist launched his campaign in Cincinnati against Australian Jordan Thompson, prevailing 7-5, 4-6, 6-3 after three hours, recording his victory number 50 on the season.
Meanwhile, Paul toppled Frenchman Ugo Humbert with a 6-1, 7-6(4) victory, setting a second matchup in as many weeks with the Spanish player.
Last week, the American took out U.S. Open defending champion in their quarterfinal clash in Toronto, pulling off the biggest win of his career.
On Monday, his strong run to the semifinals on Canadian soil saw him achieve a new career-high at World No. 13.
For his part, Alcaraz has reached his fifth Masters 1000 quarterfinal this season and 11th overall before heading to New York.
Purcell and the El Palmar native have never met one another before at tour level.