FLUSHING MEADOWS, N.Y.—All James Blake could do was shake his head. He spun around the Arthur Ashe Stadium court Monday night, tossing his headband and every tennis ball in sight to the roaring U.S. Open crowd, and a dazed smile crept across his face.

Not that the Blake was surprised by his 4-6, 7-5, 6-2, 6-3 win over No. 19 seed Tommy Robredo of Spain. No, the former Harvard All-American—he went pro in 1999, after two years with the Crimson—had already proven himself capable of a big win on Saturday, when he knocked off No. 2 seed Rafael Nadal in three commanding sets.

But as he stood there in the Monday evening dusk, his first Grand Slam quarterfinal berth finally secured, he looked stunned.

“It seemed, somehow, surreal at the time,” he said an hour and a half later, as he calmly munched a muffin in the interview room.

Also surreal to Blake is the fact that his quarterfinal opponent tomorrow will be none other than Andre Agassi, whose U.S. Open escapades Blake recalls watching as a young boy.

“Back when he had those denim shorts,” Blake laughed. “He had the long hair and the earring.”

This is the 35-year-old Agassi’s 20th year in the New York tournament—Blake himself is only 25—and for all the adoration Agassi garners as the game’s elder statesman, there’s no denying that the feel-good-story of this Open belongs to the younger of the two.

In the last year Blake has contracted the zoster virus, broken his neck and watched his father die of cancer. Not only that, but he has to relive this disastrous period nearly every day—in the sympathetic utterances from the fans, the hushed analysis of the broadcasters, and even the interview room, where he is asked if comeback performances such as last night’s are indicative of a newer, more mature perspective on the game.

But he’ll tell you they are.

Robredo held a 6-4, 5-3 lead over the admittedly flat-footed Blake before the latter reminded himself that “being down a set and a break doesn’t mean anything.”

Simple as that. Blake has been through worse and bounced back.

“And once I realized I was in it, I started moving my feet a little bit,” he continued.

Enough, at least, to reel off four consecutive games, take the second set 7-5, and knot the overall score at 1-1.

The final two sets were more traditional Blakeian fare, full of blistering groundstrokes and opportunistic netplay—and as the American began to ooze confidence, the Spaniard tightened visibly.

Blake improved his first-serve percentage from 46 in the first set to 73 in the fourth, while Robredo continued to spray unforced errors at pivotal moments, out-blundering his opponent 46-30 in the category at match’s end.