* Italy's Errani serves up rare double-bagel to advance
* Poland's Janowicz booed for serving underhand in loss (Updates with Federer winning)
NEW YORK, Aug 27 (Reuters) - Roger Federer made the perfect start to his bid for a sixth U.S. Open title when he easily won his first-round match at Flushing Meadows on Tuesday.
Defying critics who have dismissed his chances here, the Swiss master was all class as he brushed past Slovenia's Grega Zemlja 6-3 6-2 7-5 in a center court match that was held over from the previous night because of rain.
The 32-year-old Federer, winner of a record 17 grand slam singles titles, ripped 35 winners and committed just 16 unforced errors in the 93-minute win.
"I felt great," Federer said in a courtside interview.
"It's great to be back in New York, there's no doubt about it. Day session or night session it doesn't really matter when you play on Arthur Ashe Court."
Former world number one Caroline Wozniacki survived a tough examination to join grand slam winners Petra Kvitova and Ana Ivanovic in the second round.
And Italy's Sara Errani, the fourth seed, showed why she remains a contender with a rare double-bagel 6-0 6-0 win over Australia's Olivia Rogowska.
Errani showed no mercy against her 151st-ranked opponent, who only got into the draw as a lucky loser replacement following the withdrawal of Japan's Ayumi Morita, as she romped to victory in 51 minutes.
With her boyfriend, Northern Irish golfer Rory McIlroy, watching from the stands at Arthur Ashe Stadium, Wozniacki had to dig deep to beat Chinese qualifier Duan Yingying 6-2 7-5 on an action-packed second day at the U.S. National Tennis Center.
Kvitova, the 2011 Wimbledon champion, also had to work overtime before winning her clash with Japan's Misaki Doi 6-2 3-6 6-1 while a grieving Ivanovic, the 2008 French Open winner, cruised to a 6-2 6-0 win over Georgia's Anna Tatishvili.
Also a former world number one, Ivanovic arrived in New York with a heavy heart after learning about the drowning death of a childhood friend back home in her native Serbia.
Vukasin Ziramov, 25, died last week after jumping off a bridge into a river while on an outing with friends in Senta.
"It's been very sad news," Ivanovic told reporters. "It was very hard because it was almost like my relative. We grew up, and I knew him since we were kids. It's very, very sad."
Ivanovic was able to control her emotions during her brief appearance on court, cracking 16 winners in her 58-minute romp.
"It's obviously hard, more so emotionally than anything else," she said. "(But) I'm very confident with the game and the way I was playing before that."
'DAY OF SURVIVAL'
Wozniacki appeared to be in cruise control in her match against Duan, racing through the opening set in just 35 minutes before the Danish sixth seed lost her way in the blustery conditions.
The match looked to be heading to a deciding third set when Duan jumped out to a 5-2 lead before Wozniacki regained control, reeling off the next five games to avoid a repeat of her first-round exit from Flushing Meadows a year ago.
"I'm happy to be through. I think everyone that you asked today would just say it was a day of survival and a day to get through," Wozniacki said.
"It's not about being pretty. It's about just getting the job done. I did that, so I'm happy about that."
The left-handed Kvitova also had problems with the swirling wind, making 29 unforced errors before recovering from a second set lapse to ease into the second round.
"The wind was quite difficult for us but it was the same conditions for both of us," said the Czech. "It wasn't easy match for first round. It's always tricky and difficult, so I'm glad that I'm through."
Wimbledon semi-finalist Jerzy Janowicz was booed for serving underhand during a petulant display in his loss to Argentina's Maximo Gonzalez, ranked 247th in the world.
Hampered by a painful back injury, the world number 14 from Poland crashed to a 6-4 6-4 6-2 loss after arguing with the chair umpire and throwing his water bottle on the court.
"It was like being stabbed in the back by a knife," said Janowicz. "For three days I haven't been able to practice, I could barely walk. I was in really good shape before this happened and that's why I'm fricking disappointed." (Editing by Frank Pingue)