By Julian Linden NEW YORK (Reuters) - American Serena Williams and Caroline Wozniacki both face former grand slam title winners at the U.S. Open on Monday as the last major of the year enters the second week.
Williams will play Serbia's Ana Ivanovic, the 2008 French Open champion, while Wozniacki meets Russia's Svetlana Kuznetsova, who has won both the U.S. and French Open singles titles.
Williams and Wozniacki are overwhelming favorites to win their fourth round matches and eventually clash in the semi-finals but neither player is taking anything for granted.
"She's playing well," Williams said of Ivanovic. "She's incredibly fit right now. She was number one, right, at one point."
Wozniacki, the current world number one, said she was expecting a tough battle against Kuznetsova. "I haven't thought about how I'd play her," said the Dane. "She's a tough competitor. She can play very aggressive.
"It's a match that definitely would need me to play on my best."
Novak Djokovic has been almost invincible this year, losing just two matches all season and winning the Australian Open and Wimbledon titles along the way.
The Serbian has not dropped a set in his first three matches and is brimming with confidence ahead of his fourth round clash with Alexandr Dolgopolov of Ukraine. "He's a tough player (but) now we are in the second week and we are (all) expecting tough matches," Djokovic said.
Roger Federer dropped a set in his last match against Croatia's Marin Cilic but was happy just to get past a tough and experienced opponent.
"The Swiss master next plays Argentine Juan Monaco, one of four unseeded men through to the last 16.
"Monaco is a tough customer," Federer said. "He works the ball like all South Americans do, and he's got good grit and battling on every single point. It's always physical against these kind of players."
(Editing by Ian Ransom)