NEW YORK—The baseline loomed like a long-awaited finish line. Kei Nishikori, slapping his thigh as if trying to shake life into his throbbing legs, showed remarkable staying power to cross without cracking.

The U.S. Open marathon man made another four-hour run to reach a historic destination. Nishikori withstood some massive baseline blows from Stan Wawrinka—and another exhausting five-set fight—prevailing 3-6, 7-5, 7-6 (7), 6-7 (5), 6-4 in a stirring four-hour and 15-minute epic. In doing so, he became the first Japanese man to reach the U.S. Open semifinals since Ichiya Kumagae in 1918. It was an exceptional effort from the 24-year-old, who stared down 35 aces from Milos Raonic in rallying for a fourth-round win that ended at 2:26 a.m. on Tuesday morning, tying a tournament record for latest conclusion to a night match.

Aggressive court positioning and the courage to drive the ball down the line at crunch time were keys for Nishikori, who was sluggish at the start. Less than 48 hours after edging Raonic, Nishikori looked tight, tentative, and in need of a nap against the Australian Open champion. The weight of Wawrinka's heavy shots backed his opponent up behind the baseline, forcing Nishikori to counter from defensive positions.

Nishikori's issues were compounded by the fact he couldn't buy a first serve in the early stages. He served 44 percent in the opening set and paid the price when Wawrinka confronted an 83 M.P.H. serve, plastering a forehand return winner down the line to break for 2-0. Stan slid a 119 M.P.H. ace down the middle to snatch the first set in 34 minutes.

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U.S. Open: Nishikori d. Wawrinka

U.S. Open: Nishikori d. Wawrinka

Wawrinka stepped forward to pressure Nishikori's second serve in the opener, but as the match progressed, Stan retreated on return as if waiting for his slender, 150-pound opponent to wilt. It never happened. Playing without his customary baseball cap, Nishikori, pained by a quarter-sized blister on the ball of his foot, managed both his energy and emotions smartly, and began playing angles shrewdly. He won a 54-minute second set, which would turn out to be just the third longest of the contest.

Plot twists and pulsating shot-making marked a dramatic third set that saw both men earn set points with bold strikes. The 10th seed turned the match around by playing short and wide at times to drag Wawrinka off the court and set up his down-the-line drives. Zapping an inside-out forehand to break for a 4-2, Nishikori consolidated with a 117 M.P.H. serve winner for a 5-2 lead. But a nervous Nishikori failed to serve it out at as his forehand failed him, and Wawrinka won eight of the next nine points to level at 5-all.

Down 3-5 in the tiebreaker, Nishikori went soft with a bold drop shot, then coaxed successive backhand errors down the line to earn set point at 6-5. Attacking the Swiss' one-hander again, Nishikori looked stunned when Wawrinka passed him cleanly up the line to save a second set point. Stan's set point came at 7-6, but Nishikori erased it with a down-the-line backhand winner of his own—and a scream—for 7-all. Nishikori's subsequent 78 M.P.H. second serve sat up, begging to be pounded, but Wawrinka belted a backhand return long, a demoralizing conclusion to a quality 64-minute third set.

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U.S. Open: Nishikori d. Wawrinka

U.S. Open: Nishikori d. Wawrinka

After Wawrinka held for 2-1 in the fourth set, Nishikori took a medical time-out to have his blistered foot re-taped. Both men dominated on first serve—Wawrinka lost only three points on first serve and Nishikori permitted just four first-serve points—in a set devoid of breaks. At 5-all in the breaker, Wawrinka drew a backhand error for set point and buried a body serve to level the match. Coach Magnus Norman recoiled in his seat like a man who had just survived a bumpy landing after a cross-country flight.

Teetering on the edge of cracking, Nishikori stared down double break point in the third game of the decider. He saved the first attacking behind a backhand, and the second with a forehand down the line. A spinning backhand volley ended a hard-fought hold for 2-1. Coach Michael Chang, clad in a black baseball cap bearing a pizza parlor logo, jumped from his seat exhorting his charge with a double fist-pump.

Amazingly, it was Wawrinka who wilted in the end, dumping his eighth double fault down 4-5 to face double match point. When Stan buried a forehand into net, Kei stared at his box, in exhaustion and elation, looking too tired for an extensive celebration—or even a short walk. He managed a small smile to end a major struggle, shook Wawrinka's hand, and moved forward.

"I started a little tight, but my body was okay," Nishikori told ESPN's Brad Gilbert afterward. "I don't know how I finished the game, but I'm very happy I did."