NEW YORK (REUTERS) - Defending champion Andy Murray fears Stanislas Wawrinka has developed the confidence required to cause an upset in their US Open quarter-final at Flushing Meadows on Thursday.
Murray owns an 8-5 all-time record against the ninth-seeded Swiss but he lost to Wawrinka in their only meeting this year, a last-16 clash at the Monte Carlo Masters.
The British third seed said once a player of Wawrinka's calibre reaches a certain level of technical competence, confidence becomes the most important quality.
"If someone goes on the court not thinking they can win against you, then the match is as good as done," said Murray. "When they believe they can win, in an individual sport when you're just playing against one person, then you can cause an upset. It's as simple as that.
"Stan has improved quite a bit this year. He hasn't changed any of his strokes, technically. You'd expect most of it to be confidence."
Murray and Wawrinka will meet in Thursday's day session on Arthur Ashe Stadium while the night session features top-seeded Novak Djokovic and 21st-seeded Russian Mikhail Youzhny in the day's other quarter-final.
Serbian Djokovic claimed he was in stellar form after he demolished Spaniard Marcel Granollers 6-3 6-0 6-0 in the fourth round on Tuesday.
"I had some great matches in the past that I can compare this one to," he said. "Definitely the second and third sets have been some of the best tennis that I've played on Arthur Ashe in my career."
"It all comes at a great time for me."
Wawrinka said his self-belief escalated when he pushed Djokovic to the limit in a five-set match at the Australian Open in January.
"Andy is a really good player and we practice a lot together," he said. "It's usually a tough match but I like to play him. I like his game.
"If I can start well, if I can stay with him, I have a good chance. I need to start the match well to show him I'm going to be there."