Andy Murray has admitted that winning against Stanislas Wawrinka in the US Open quarter-finals will be "very tough".
The defending champion defeated Denis Istomin 6-7(5) 6-1 6-4 6-4 in the fourth round to reach the last eight.
"Wawrinka will be very tough," BBC Sport quotes him as saying. "He has played great tennis the whole year, is ranked eight or nine and will be higher after this.
"He's very talented. We played once before on this court when we were much younger and a lot has changed since then, but it will be tough."
Wawrinka beat Tomas Berdych in four sets in the fourth round.
