By Simon Cambers
NEW YORK, Sept 1 (Reuters) - Japanese teenager Kei Nishikori's ground-breaking U.S. Open run came to an end in the fourth round on Monday with a 6-3 6-4 6-3 defeat by fellow young gun Juan Martin Del Potro of Argentina.
The 18-year-old, the first Japanese man to reach the last 16 in the U.S. Open since 1937, was outgunned by Del Potro, the 17th seed who extended his winning streak to 23 matches.
"I was a little tired, obviously, and mentally more tired," Nishikori told reporters.
"At the same time, he was playing really well. He didn't miss any balls. He was just playing well. I was just waiting (for) his mistake (and) I couldn't use my forehand."
Del Potro, 19, will now play sixth seed Andy Murray of Britain.
Nishikori, in just his second grand slam event, had beaten fourth seed David Ferrer in the previous round but found Del Potro, who has won his past four tournaments, too tough.
The Japanese led 3-0 in the first set but Del Potro won six straight games to take it before moving ahead 5-2 in the second.
Nishikori gave the Japanese fans inside the Louis Armstrong Stadium hope when he broke back for 5-4 and when he led 40-0 on serve in the next game, it looked like he would level at 5-5.
But Del Potro won five straight points to take the set and then broke twice in the third to reach the quarter-finals of a grand slam event for the first time.
In addition to his win over Ferrer, Nishikori's run to the last 16 included a win over 29th seed Juan Monaco in round one.
Shuzo Matsuoko is the only Japanese man to reach the quarter-finals of a grand slam event, at Wimbledon in 1995, and Nishikori said his countryman was an inspiration to him.
"He taught me so many things when I was young," he said. "I was like 12, 13. And, yeah, I'm trying to beat his career.
"(I also look up to) Roger (Federer) and (Rafael) Nadal. I like how Nadal plays, and Roger, too. He can do everything and play so smart. That's what I'm trying to be."
A delighted Del Potro said he had been concerned when he fell 3-0 behind in the first set.
"He started better than me, but after I broke, his serve and then I started to play my game and it was a little easier," he said.
"But I did my shots, my game, and finally I won the match in three sets.
"Today he didn't play too good. I'm happy to be in the quarter-finals, but it's a good tournament for him, also."
(Editing by Claudia Parsons)