The 19-year-old Alcaraz triumphed 6-4 3-6 6-4 4-6 6-3 in a match that finished at 2:23 a.m. local time — just three minutes shy of the tournament’s latest finish.
Alcaraz, the highest-ranked player left in the men’s draw, will contest his second US Open quarterfinal when he takes on Jannik Sinner on Wednesday.
“It was unbelievable,” Alcaraz said in his on-court interview. “After the fourth set, I had a lot of opportunities at the end, lost the fourth set.
“It was tough for me to come back in the fifth set, to stay strong mentally, but … the energy that I received (from the crowd) today made me win.”
Against Cilic, in a match that lasted close to four hours, Alcaraz led after the first and third sets but couldn’t put his Croatian opponent away until the fifth.
He went a break of serve down at the start of the match but fought back to win the next four games and take the first set.
There was a similar story in the second set as Alcaraz broke early before Cilic reeled off four consecutive games and held serve to level the match.
The third set stayed on serve until Alcaraz broke at 5-4, but his 2-1 lead was cut when Cilic broke late in the fourth — despite the teen having break points of his own throughout the set.
As the match carried on well past midnight, Alcaraz seized the advantage with a break to love at 2-1 and sealed the victory when Cilic pummeled a forehand into the top of the net.
This was by far the toughest test of the tournament so far for Alcaraz, who had not dropped a set prior to facing Cilic.
He next faces another rising star in 21-year-old Sinner — the Italian, like Alcaraz, hoping to reach a grand slam semifinal for the first time.