US Open PHOTOS: Radwanska falls; Murray, Nishikori march into quarters

Radwanska falls to teenager Konjuh

Croatian teen Ana Konjuh upset fourth seed Agnieszka Radwanska 6-4, 6-4 on Monday to claim the last women's quarter-final spot at the US Open.

The 18-year-old, who won the 2013 US Open juniors title after claiming the Australian Open juniors crown, lost her first service game but did not yield serve again.

She cracked 38 winners, including six aces, against the Pole, who has never gone beyond the round of 16 in five attempts at Flushing Meadows.

The power-hitting teenager, who became the first Croatian woman to reach the quarter-finals at a US Open, avenged a second-round loss to Radwanska at Wimbledon, where she held three match points before falling 9-7 in the third set.

"It's been an incredible night," Konjuh said in an on-court interview.

"This time I got revenge, so I'm really happy." The victory lifted Konjuh to a last eight match against 10th-seeded Karolina Pliskova of the Czech Republic.

Murray makes quick work of Dimitrov on Labor Day

It may have been the Labor Day holiday in the United States on Monday but Andy Murray came ready to work, demolishing Grigor Dimitrov 6-1, 6-2, 6-2 to storm into the quarter-finals of the US Open.

With the year's final grand slam entering its second week, Murray posted a statement win by dispatching the 22nd-seeded Bulgarian in a speedy two hours to signal he is well prepared to extend a dream summer that has included a second Wimbledon title and a second Olympic gold medal.

Dimitrov, who ended Murray's title defence at the 2014 Wimbledon and came out on top in their last meeting earlier this season in Miami, represented a potentially tricky opponent for the second-seeded Scot.

But Murray was firing on all cylinders, rating the match that included a career high 141-mph ace to close out the first set as one of his best this season.

"It was definitely up there," said Murray. "I played very well today with very few unforced errors and made it very tough for Grigor.

"That would be the fastest serve I've hit, the other best serve was here at the U.S. Open around 138 but I have never hit over 140.

"I think that was lucky, I only did it once."

Nishikori topples giant Karlovic, to reach quarters

Kei Nishikori cut down the towering Ivo Karlovic of Croatia with surgical precision in a straight-sets win that lifted the sixth-seeded Japanese to the quarter-finals.

Nishikori, the US Open runner-up two years ago, beat Karlovic 6-3, 6-4, 7-6(4) in the final main draw singles match at Louis Armstrong Stadium, which will be replaced by a new stadium with a retractable roof for the 2018 tournament.

It was a David and Goliath clash of physical opposites, but the 6-foot-11 (2.11 m) Karlovic's tennis game, apart from his prodigious serve, was overshadowed by the skills of Nishikori, more than a foot shorter at 5-10 (1.78 m).

The quick-footed Nishikori made 37-year-old Karlovic look like he was standing still, which he often was, as he raced to victory in two hours to earn a berth into the last eight.

"It's never easy against someone like Ivo, he has a great serve," Nishikori said about the Croat, who crushed a U.S. Open record 61 aces in his five-set, first-round win over Lu Yen-Hsun of Taiwan.

"I made a lot of returns of first and second serves and was happy with my return game today."

Nishikori will play either second seed Andy Murray of Britain or 22nd seed Grigor Dimitrov in the quarter-finals.

"I've been playing well, especially today," the Japanese said. "I think I played one of (my) best matches so far. So will try to recover well and play (more) good tennis next round."

Karlovic hammered in 21 aces but when his first serves were not finding the mark, Nishikori pounced.

The Croat led all players with 99 aces. Nishikori broke Karlovic twice and was credited with 44 winners against only seven unforced errors.

"I was able to get the break, first and second set, early. So that makes me a little more relaxed," said Nishikori.