Andy Murray feels he is close to hitting top form after an impressive victory over Jo-Wilfried Tsonga last night set up a blockbuster US Open quarter-final against Novak Djokovic.

Murray had not beaten a top-10 player since his emotional victory over Djokovic to win Wimbledon more than a year ago but played a very fine match in the New York heat to defeat ninth seed Tsonga 7-5 7-5 6-4.

It maintained the Scot's impressive record of having reached at least the quarter-finals of every grand slam tournament he has played in for the past four years.

The performance was certainly a step up from Murray's first three matches in New York, with his only two lapses coming at the start of the second and third sets and quickly corrected.

The eighth seed said: "I don't feel like I'm that far away from playing my best tennis. It's still obviously a long way from trying to win the tournament but it's only nine sets now, three matches, and I'll just try and take it one set at a time.

"I'm maybe five, six days away from potentially winning another grand slam" - Andy Murray

"I'm maybe five, six days away from potentially winning another grand slam. I know there is a lot of work to be done between now and then.

"But I'm putting everything out there on the courts, it's the last slam of the year. I hope I can play well."

There was a lot to like about Murray's performance against a player who last month beat Djokovic, Murray, Grigor Dimitrov and Roger Federer back to back to win the Masters title in Toronto.

Both his first and second serves had more pop on them, he was aggressive and consistent off both forehand and backhand wings and played a few inspired shots, in particular a running cross-court winner off a Tsonga smash in the third set.

Murray had been keen to play down the top-10 statistic in the build-up to the match, and it was a similar story afterwards, but it certainly appeared to be a huge win.

The 27-year-old said: "Winning matches does build confidence. I played against some very good players this year and lost a few close matches.

"How big is it? I don't really know, to be honest. I hadn't thought that much about it. But I'll just try and take the confidence that I gained from a win like this into the next round, and hopefully put on another good performance."

Another encouraging factor was that Murray handled the hot and humid conditions well, with no recurrence of the cramp issues that affected him in his first-round match against Robin Haase.

The Scot struggled a bit with pain in his stomach, which he admitted was from drinking too much as a pre-emptive measure.

"It was obviously extremely humid," he said. "But sometimes it depends on the day. I don't know if the conditions today were harder than they were last Monday. They felt harder.

"Last Monday I just had a terrible day. I don't know exactly what happened. I don't know why it happened. Obviously a lot of players have struggled since then."

Meanwhile, Australian Open champion Stan Wawrinka fought off two set points in the pivotal third set on the way to a 7-5 4-6 7-6(7) 6-2 win over Spain's Tommy Robredo to reach the quarter-finals.

After splitting the first two sets, the third-seeded Swiss trailed 6-4 in the third-set tiebreaker but battled back to win 9-7 and closed out the victory over the Spanish 16th seed in the fourth set.

"It was a really tough battle today. It's always tough to play against him, he is always fighting," said Wawrinka, who showed his competitive fire with a dive into the stands during a point in the tight confines of Louis Armstrong Stadium.

Wawrinka will play 10th-seeded Kei Nishikori in the quarter-finals, after the Japanese player outlasted fifth seed Canadian Milos Raonic 4-6 7-6 (4) 6-7 (6) 7-5 6-4 in a match that equalled the record for the latest match ever played at the year's final grand slam.

Raonic and Nishikori stepped onto the court for the last match of the evening at the Arthur Ashe Stadium, and exited four hours and 19 minutes later in the wee hours of Tuesday morning at 02.26 having contested what was the joint latest finish ever at the US Open.

The finishing time matches two other marathon contests played at Flushing Meadows by Philipp Kohlscheiber and John Isner in 2012 and Mats Wilander and Mikael Pernfors in 1993.