US Open, men's quarter-finals: Andy Murray to take on Kei Nishikori, Juan Martin del Potro up against Stanislas Wawrinka

Second seed and 2012 champion Andy Murray crushed Bulgaria`s Grigor Dimitrov 6-1, 6-2, 6-2 to reach a sixth US Open quarter-final Monday, clocking up a personal fast serve record in the process.

The 29-year-old reigning Wimbledon and Olympic champion stormed to an impressive seventh win in 10 meetings against the 22nd-seeded Dimitrov.

From the moment Murray broke for a 3-1 lead in the first set, on the back of a lung-busting 32-shot rally, the outcome of the Arthur Ashe Stadium clash was never in doubt.

For good measure, Murray sent down his fastest ever serve of 141mph (226.9 km/h) at the end of a set where he allowed Dimitrov just five points on his service.

"I once hit a 145mph serve in San Jose but they recalibrated the machine the next day so it didn`t count. Tonight was the first time I have gone above 140," said the 29-year-old.

"It was lucky and I doubt I`ll ever do it again."

Dimitrov broke only once in the match in the fourth game of the second set but it was a brief respite as Murray quickly reclaimed it before taking 10 of the next 12 games to seal the rout.

Dimitrov committed 43 unforced errors as Murray set up a last-eight clash with Japanese sixth seed Kei Nishikori, the 2014 runner-up who downed 37-year-old Ivo Karlovic 6-3, 6-4, 7-6 (7/4).

Karlovic fired 21 aces in the contest but Nishikori`s 44 winners and a miserly seven unforced errors proved the key.

"It`s never easy facing someone serving like Ivo, never easy to return those kind of serves," said Nishikori.

"But I tried to stay down. I have been returning well, so that also helped today. I think I played one of the best matches -- serve, return, groundstrokes."Juan Martin del Potro, the 2009 champion, became the lowest ranked player in 25 years to reach the quarter-finals.

The 142nd-ranked del Potro was leading eighth-seeded Dominic Thiem 6-3, 3-2 when the Austrian retired with a right knee injury.

Del Potro will face fellow Grand Slam title winner Stan Wawrinka for a semi-final spot.

Wawrinka, a former Australian and French Open champion, reached a fourth successive quarter-final in New York with a hard-fought 6-4, 6-1, 6-7 (5/7), 6-3 win over Illya Marchenko, the world number 63 from Ukraine.

Del Potro is playing just his second Grand Slam event since the 2014 Australian Open.

Earlier this year, he was on the brink of retirement after undergoing three wrist surgeries.

"You never want to win a match like this. I wish Dominic a quick recovery as he has a great future," said 27-year-old del Potro, the lowest-ranked man in the last-eight since Jimmy Connors, at 174, went all the way to semi-finals in 1991.

The 23-year-old Thiem had needed five sets to beat Australia`s John Millman in the first round and four to get past Pablo Carreno Busta of Spain in the last 32.

"I couldn`t bend my knee too much in the last three days so I was handicapped a little bit," said Thiem playing his 70th match of the year.

Should del Potro make the semi-finals he would be the lowest-ranked man to do so at a Grand Slam since 237th-ranked Vladimir Voltchkov at Wimbledon in 2000.

No man ranked as low as del Potro has ever made the final of a major.

Having saved a match point in an epic triumph over Dan Evans in the last round, Wawrinka, a semi-finalist in 2013 and 2015, cruised through the first two sets against Marchenko.

The Ukrainian, who arrived in New York on a seven-match losing streak, battled back from 2-4 and 3-5 down in the third set to force a tiebreaker.

He was a break to the good at 2-1 in the fourth before Wawrinka, reinvigorated by a violent racquet smash courtside, recovered to win five of the next six games.

"It was tough to lose that third set but I am looking at the big picture," said 31-year-old Wawrinka, whose 49 winners helped offset his 41 unforced errors.

The first two quarter-finals take place on Tuesday when defending champion Novak Djokovic faces France`s Jo-Wilfried Tsonga and Lucas Pouille, the conqueror of Rafael Nadal, tackles French compatriot Gael Monfils.