By Steve Keating

NEW YORK (Reuters) - Second seed Angelique Kerber was first through to the semi-finals of the U.S. Open on Tuesday when Roberta Vinci was tripped up by a foot fault and tumbled out of the year's final grand slam falling 7-5 6-0.

The match had looked an intriguing one pitting Kerber, who is eyeing Serena Williams's world number one ranking, against the 33-year-old Italian Vinci, who discovered a fountain of youth at Flushing Meadows last year when she reached the finals.

But there would be no return trip for Vinci as a confident Kerber hung tough during a rollercoaster opening set then delivered a devastating knockout punch in the second to reach the U.S. Open semi-finals for the first time since 2011.

"The key of the match was the first set," said Kerber, who can supplant Williams atop the world rankings if the American fails to reach Saturday's final. "I mean, it was really close and tough.

"She's a tough opponent with her slice. I was trying to stay in the match and not thinking that I'm the break down.

"Just going for it."

Three times Vinci would break Kerber in the opening set and three times she would allow the German to immediately break back, including once while serving at 5-4 for the set.

But it would be Kerber claiming the decisive break to win the set when Vinci was called for a foot fault on her second serve before sarcastically applauding the line judge as she walked off the court.

The Italian never recovered and Kerber blasted through the second set in 24 minutes to clinch her semi-final spot.

"Bad luck for the first set," said Vinci. "I had so many chances.

"Probably I was playing better than her the first set but she's a great player.

"So I lost the first set, and then the second set she started to play better than the first set. I was a little bit down."

After last year's magical run to the final, which included one of the biggest grand slam upsets of all-time when she beat Williams in the semi-finals, Vinci's expectations and ranking had both risen dramatically on her return to Flushing Meadows.

A year ago oddsmakers had set the unseeded Vinci as a 300-1 longshot to win the season's final grand slam but this year she arrived as a legitimate contender and seeded seventh.

But while Vinci was able to deny Williams a chance to play for a rare calendar-year grand slam in New York last year, she was unable to find a way past Australian Open champion Kerber, who will next face either Denmark's Caroline Wozniacki or Latvian Anastasija Sevastova.

"This morning in my mind I said, 'ok try your best'; tough opponent like Serena last year," said Vinci. "You have nothing to lose. Play your game. Try to enjoy.

"She's number two in the world. Maybe number one, so she's in confidence.

"I knew it was tough, tough to win against her."

(Editing by Frank Pingue)