Extra gear needed if Murray is to defend his crown

The third seed has not looked in any danger of going out over his first four matches, but neither has he found anything like his best form.

The form he displayed in beating Denis Istomin in four sets on Tuesday night will not be good enough if he is to defeat confident Stanislas Wawrinka in the quarter-finals tonight.

While Murray was struggling to see off world No 65 Istomin 6-7 (5/7) 6-1 6-4 6-4, Wawrinka was continuing the best season of his career by knocking out fifth seed Tomas Berdych.

It is not the first time the Scot has found it difficult to get through the early rounds, and his recent grand slam record suggests he knows how to step it up when it matters.

Since losing to Wawrinka in the third round in New York three years ago, Murray has reached 11 consecutive grand slam quarter-finals – he missed the French Open this year through injury.

He has lost only once in the quarters in that time, and that came on clay at Roland Garros.

Murray said: “Hopefully, I’ll start playing a little bit better from now on.

“Sometimes the first week of a slam, I’ve played very well. Sometimes you maybe feel like there’s something to lose a little bit.

“But I’m in the quarter-finals of a slam. It’s not an easy thing to do, and the matches are going to get tougher. I’ll have to up my game.”

Murray’s serve has not been the weapon he would like it to be, although he has been broken only four times in the tournament.

The speed of his first serve has fluctuated from 136mph to less than 90mph, while he has served an average of only five aces per match.

The weather has certainly been a factor, with the intense humidity of Murray’s third-round win over Florian Mayer being replaced by cool and windy conditions on Tuesday night.

Murray said: “I think I would like to be serving a little bit harder, but if you’re not comfortable on serve you’re going to take some pace off of it.”

Wawrinka looks supremely confident having returned to the top 10 this summer for the first time since 2008.

The 28-year-old is good friends with Murray and the two often practise together.

Murray said: “I’ll need to play a good match. He’s improved this year quite a bit. Some of the things he didn’t do so well before he’s improved.

“He has a very good backhand. He serves well. He’s solid up at the net. He makes a lot of returns. He makes it very tricky for you. He’s a top player. It will be a very tough match for me.”

Wawrinka has always been a potential danger to the top players with his fine shot-making, particularly his one-handed backhand, but his results have been erratic.

This year he has shown more consistency and produced some stunning displays, none better than in the epic match he played against Novak Djokovic in the fourth round at the Australian Open, which he lost 12-10 in the fifth set.

Wawrinka highlighted the title he won in Portugal in May as a key moment and said of his improved form: “It’s confidence, for sure.

“I had a tough beginning of the year but at the same time it was really positive for me because I was playing great tennis.

“I think the title in Portugal against David Ferrer in the final helped me a lot with my confidence to reach the next level. Since then, I’m feeling really good on the court.

“This is for sure my best year. I think I’m playing my best tennis ever.”

In the women’s draw, Flavia Pennetta continued her remarkable US Open by defeating fellow Italian Roberta Vinci to reach her first grand slam singles semi-final.

The 31-year-old had made the quarter-finals in New York three times before but was well off the radar before the tournament began. Initially, Pennetta was not even ranked highly enough to get into the main draw but crept in after a withdrawal.

The world No 83 is a former top-10 player but missed this tournament last year and the following six months because of a wrist injury.

Pennetta had reached the last eight without dropping a set, beating fourth seed Sara Errani in the second round, former champion Svetlana Kuznetsova in round three and in-form Simona Halep in the last 16.

She was always the dominant player again yesterday, her extra power giving her the edge over 10th seed Vinci, and she eased to a 6-4 6-1 victory.

Vinci had been the winner on Tuesday when the pair met in doubles but, after pulling back from 2-4 to 4-4, she lost her serve again and the set.

The second set was much more one-sided, and Pennetta will meet either second seed Victoria Azarenka or Daniela Hantuchova in the last four.

Britain’s last representative went out of the doubles as Dom Inglot and his Filipino partner Treat Huey were beaten in the quarter-finals 7-5 6-3 by 10th seeds Ivan Dodig and Marcelo Melo.