NEW YORK -- Kei Nishikori is making history with every swing of his racket, and that can be tiring.
After his latest triumph, a grueling five-set endurance test against 4th-seeded Spanish grinder David Ferrer, the slight 18-year-old limped into the interview room, grimaced as he sat down, ran one hand through his spiky hair and put his head down on the table for a moment to collect himself.
He spoke softly and slowly, pulling the words up from the bottom of a deep well of emotion.
"Right now I'm very happy,'' said Nishikori, who cramped so severely during the match that he required a near-full-body massage at one point. "Yeah, that's the only word I can say now. And, you know, I couldn't give up the fifth set … I tried to think, 'I am playing David, he's No. 4 in the world, and playing five sets with him.' I felt like kind of happy and [began to] think more positive. Yeah, that's why I think I could fight through everything.''
Nishikori's talent, aptitude, adaptability and spirit have made him a barrier-breaker in several ways. He is the first Japanese man to reach the round of 16 in a Grand Slam event since Shuzo Matsuoka did it in 1995 at Wimbledon, and the youngest man to advance this far at the U.S. Open in 10 years.
He and fourth-round opponent Juan Martin Del Potro of Argentina, who turns 20 later this month, are the first teenagers to go this deep in this tournament in tandem since two precocious kids named Andy Roddick and Tommy Robredo cracked the ceiling together in 2001. The Nishikori-Del Potro match is scheduled for early Monday evening to accommodate Japanese television, where his matches have been scoring monster ratings.
The upper echelons of men's tennis have largely been a grown man's world for the last few years. Only two men in the top 50 are under 20 years old -- towering 17th-seed Del Potro, who will put a 22-match win streak on the line against Nishikori, and No. 24 Marin Cilic of Croatia. Size, speed and muscle matter. Few teenagers have been able to run with the big dogs -- with the notable exception of Rafael Nadal, whose early physical maturity helped him win major tournaments before his 20th birthday.
At 5-foot-10 and a wiry 150 pounds, Nishikori has the opposite body type from the world No. 1, but he clearly has developed core strength along with his crowd-pleasing leaping forehand. He became a permanent resident at Nick Bollettieri's famous Bradenton, Fla., academy at age 13, recruited for his scrappy game and obvious potential and financially sponsored by Masaaki Morita, the tennis-mad retired CEO of the Sony Corporation whose foundation pays for top Japanese prospects to visit the academy every year.
Not all of them stick, but Gabriel Jaramillo, the academy's director of tennis, was confident that Nishikori would after auditioning him at age 12. "He had an atrocious serve, but I liked how fast and fit he was and how aggressively he played the points,'' Jaramillo said.
Nishikori, who grew up in Shimane, near Hiroshima, spoke no English when he arrived in the United States. He trained with a Japanese coach and other Japanese players for the first two years he was at the academy, and saw his parents only once or twice a year. In those days, it was homesickness, not dehydration, that turned his legs to jelly.
"I was so nervous. I was like scared of everything, all the American people,'' said Nishikori, who to this day is not crazy about American food.
At 15, Jaramillo began weaning him from that sheltered lifestyle. "He was smart enough to realize that's what he needed,'' said the Colombian-born administrator.
Nishikori roomed with young Americans and began to see the wider world while touring the junior circuit, where he rose to No. 6 in the world. He began playing backwater professional tournaments in 2006, laboring on the clay courts of South America and hard courts in the United States, honing his skills in obscurity. He won a handful of ATP-level matches last year and reached the finals of two Challenger events.
Obscurity ended officially last February, when Nishikori blazed through the draw in Delray Beach, Fla., and toppled then-No. 12 James Blake in the final to win his first ATP title. By then, he'd turned pro, bought a house of his own near the academy and gone solo with his own coach, Glenn Weiner, part of the Bollettieri stable and an academy product himself.
Weiner, who was born in South Africa, has seen plenty of young players come from overseas and flame out. He's impressed with Nishikori's self-belief.
"Obviously, he had a dream in mind, and he was willing to be uncomfortable at times to achieve it,'' Weiner said.
"Tennis is a sport where you fight yourself. There are so many ways to sabotage success, but I don't think he questions himself -- he tries to figure out a way to win. He reminds me of Nadal that way.''
During his match with Ferrer, Nishikori consulted a notebook with helpful reminders he shyly revealed after some demurring. " 'Stay calm','' he said. " 'Don't get pissed off too much.' Kind of like that.'' Against Del Potro, he'll have to keep his cool in the face of a powerful serve and one of the hottest players on tour.
Speaking of Nadal, Nishikori managed to win a set off the Wimbledon champion on grass in the lead-up Queens Club tournament in London. He is respectful of but unintimidated by the stars of the game, having practiced with top players like Tommy Haas of Germany and Belarus veteran Max Mirnyi since he was a lad.
He might be among them sooner than most people thought. Currently ranked No. 126, Nishikori will enter the top 100 thanks to his run here -- he's projected to get to at least No. 85 -- and while he has a lot of weapons, surprise will no longer be among them.
Bonnie D. Ford covers tennis and Olympic sports for ESPN.com. She can be reached at email@example.com.