Never doubt the word of Andy Murray.

The 27-year-old had promised to deliver his first win over a top-10 player in more than a year and yesterday he did just that as he beat ninth seed Jo-Wilfried Tsonga 7-5, 7-5, 6-4 to take his place in the quarter-finals of the US Open.

It was a hugely encouraging performance from the No.8 seed, perhaps his best of the year, as he coped well, physically and mentally, with blistering heat to reach the last eight.

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Murray's relief and satisfaction were clear to see as he held his arms up to the sky and then punched the air after Tsonga's final backhand went long to seal victory.

"It was extremely tough conditions, very humid and windy," Murray said. "It started to cool down a bit at the end but the first two sets were very long, mentally quite draining and I am just glad I managed to get through. The third set was tougher, he had some break points to go up a double break and from one end, it was windy and easier to break.

"So when I got the hold from 2-0 down, the momentum was with me. Jo serves big and thankfully he missed a few at the end."

For all the talk of a disappointing year, Murray is the only man other than Djokovic to have reached the quarter-finals or better of all four grand slams. There were plenty of tough moments, as there were always likely to be, with Tsonga 4-2 up in the second set and 2-0 up in the third before Murray hit back.

His movement, so hampered by cramps in round one, looked close to being back at its best once more, his returns were sharp and his serve did early damage to put him on track.

When he dropped serve at the start of the third set, Tsonga must have harboured at least some hope of a bold fightback and he forced three break points to extend his lead to 3-0.

But gathering himself for another effort, Murray held serve and then broke back to get back on serve.

One outrageous forehand winner - slapped on the run after chasing down a Tsonga smash - emphasised the swing in momentum and, at 5-4, as the Frenchman finally wilted, Murray broke again to seal victory.

What will have particularly pleased Murray was the way he clinched both the first and second sets, playing aggressively and taking the time away from Tsonga.

In the first set it was a brilliant backhand return, followed by a swift move to the net to put away the backhand volley; in the second, a stunning forehand gave him the break back for 4-4 and he went after a forehand return on set point to force an error from Tsonga and the break to double his lead.

The third set was just as tough as Tsonga broke in the opening game but, at 2-0 down, Murray dug deep to hold serve against the wind, saving three break points in the process.

Changing ends, there was suddenly a spring in his step and he broke back immediately and from then on, was always in charge.

Things will get even more difficult from here on, though, with the world No.1 Djokovic waiting in the last eight, a repeat of the 2012 final here and last year's Wimbledon final, both of which Murray won.

"It'll be a tough match," the Scot said. "We've had a lot of long ones, normally a lot of rallies, we played a lot of long ones here a couple of years ago and I have great memories of that match. It's going to be very hard but if I play well, I'll have a chance."

Djokovic came through his first test of the tournament as he saw off Kohlschreiber 6-1, 7-5, 6-4, saving a set point in the second set with a stunning forehand pass, willing the crowd to get involved.

"It was one of the turning points of the match," the Serb said. "If he won that point, won the second set, it's pretty even. So it was a big point, and I just tried to get the crowd engaged."

Having come into the tournament with doubts about his form and motivation - despite having won Wimbledon - Djokovic has turned on the after-burners here and is relishing his shot at an eighth grand slam title.

"I think it's great that I have been playing some really good tennis, really high quality so far, but it's normal to expect that I'm going to have tougher opponents as the tournament goes on," he said.

"Quarter-finals against Murray is a very tough draw. But in order to get far in this tournament, I have to win against the best players in the world."

Djokovic said he was looking forward to locking horns with Murray again, having come off second best in the Olympics then in New York in 2012 and at Wimbledon last summer.

"He's already been a grand slam champion, Wimbledon, US Open, Olympic Games so he has a lot of success behind him and a lot of experience behind him," Djokovic said.

"He knows how to play on Centre Court here. We've had some great matches, including the final in 2012 in five sets. I'm going to have to play at my highest level in order to advance."