Murray moves into quarter-finals at US Open
Last year's US Open champion moves into quarter-finals, where the competition gets tougher as he faces the ninth-ranked Wawrinka
So far in his first grand slam title defence, Andy Murray needed to beat players who are ranked 47th, 49th, 65th and 81st.
He's well aware things probably will get a tad tougher the rest of the way at the US Open, starting with No 9 Stanlisas Wawrinka in the quarter-finals.
"Hopefully I'll start playing a little bit better from now on. I mean, sometimes the first week of a slam, I've played very well. Sometimes you maybe feel like there's a little bit something to lose," said Murray, who is 30-2 over the past five major tournaments.
"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," he said. "I'll have to up my game."
In a 6-7 (7-5), 6-1, 6-4, 6-4 fourth-round victory over Denis Istomin of Uzbekistan on a windy night, Murray found reasons to yell at himself for mistakes.
He faltered late in the first set, dropping four points in a row. He also flubbed what should have been a simple volley after Istomin's between-the-legs shot late in the second. There was more shakiness from Murray in the third set, in which he went up a break, only to lose serve to make it 3-3. But he broke right back for a 4-3 edge, and was back on his way.