New York: Petra Kvitova is proof that practice does not always make perfect after the Czech revealed she is spending only half as many hours training due to illness.
The two-time major champion announced she had been diagnosed with mononucleosis earlier this month, a virus which can cause fatigue, fever and body aches.
It has caused Kvitova to cut her practice hours dramatically in the build-up to the US Open but she has still reached her first quarter-final in New York after beating Britain's Johanna Konta 7-5, 6-3.
"I'm not practising that much, I'm kind of a little bit quiet, not really doing these normal kind of things, not really expecting anything from myself," Kvitova said.
"I didn't want to put that pressure that much on me. The number of hours practising is very low. When I find out I have mono, I was really cutting the hours down.
"I was practising only once a day, maybe hour, and in the afternoon have 30 minutes of fitness. It's really kind of half, let's say."
Kvitova, who is seeded fifth in Flushing Meadows, has played semi-finals at both the French and Australian Opens and is a two-time Wimbledon champion.
Success at the US Open, however, has so far eluded the 25-year-old who lost in Round 3 in the previous two years and had never been past the Last 16. Kvitova will now play Flavia Pennetta after the Italian beat Australian Samantha Stosur in straight sets.