Andy Murray knows he must play better if he is keep the defence of his US Open title on track.
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.
And the display he gave in beating Denis Istomin in four sets on Tuesday night will not be good enough if he is to defeat a confident Stanislas Wawrinka in the quarter-finals.
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 only an average of five aces per match.
The weather has certainly been a factor, while 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.”
It took most of the first set for both Istomin and Murray to get their eye in in the swirling breeze.
Istomin is unique among top-level men in being coached solely by his mother Klaudiya, and he is a talented shot-maker who has given a lot of the leading players some trouble.
The pair had met once before, in Brisbane earlier this year, with Murray winning in two close sets.
It was Istomin who struck first here, breaking for 5-3, but back came Murray immediately and he looked poised to win the tie-break at 5-3 ahead.
The Scot faltered, though, one great piece of play from his opponent and three unforced errors giving Istomin the set.
Murray made a much better start to the second, a fizzing forehand helping him break in the second game, and with an hour and a half of the match gone he was level.
Istomin did, though, have the satisfaction of winning a point with a between-the-legs shot, Murray somehow missing a simple volley.
The third set was a mixture of the very good and the sloppy from Murray, who broke for 2-1 only to give his advantage away three games later.
His forehand was beginning to really fire and he finally converted a fourth chance to break again in the seventh game.
It remained a struggle in the fourth set and In the end it was a huge stroke of luck that helped Murray clinch the crucial break of serve in the ninth game.
Murray had seen one break point come and go when he framed a forehand high into the night sky. Amazingly it dropped in and the third seed capitalised before serving out the victory in three hours and seven minutes.
Istomin certainly entertained the crowd, doing keep-ups at one stage and attempting three shots between his legs.
The first of those came at a tight moment in the first set when he put the ball in the net despite being in a great position to hit a conventional winner.
Murray said: “He had a pretty easy shot. I personally wouldn’t try something like that at that stage, but he did. It was a bit surprising.”
Meanwhile 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 practice 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.”