* Azarenka-Ivanovic match moved to Tuesday
* Thunderstorms wipe out five hours of play (Updates with matches cancelled)
NEW YORK, Sept 2 (Reuters) - Nearly five hours of play was lost to rain at the U.S. Open on Monday, forcing tournament organisers to cancel dozens of matches and issue an revised playing schedule.
Only one of the eight singles matches scheduled for Monday was completed before thunderstorms arrived just after midday ET (1600 GMT).
Play did not resume until just before 5 p.m. ET (2100 GMT), by which time officials had to change the schedule to cope with the backlog of matches.
A total of 37 matches, mostly doubles and juniors, were postponed until Tuesday. The only main draw singles match that was held over was the highly-anticipated center court clash between world number two Victoria Azarenka and Ana Ivanovic.
Roger Federer's fourth round match against Tommy Robredo was moved from the Arthur Ashe Stadium to Louis Armstrong Stadium, marking the first time he had played on the smaller court since 2006.
Before the rain arrived, Roberta Vinci managed to beat her opponent and the foul weather in the nick of time to charge into the quarter-finals.
Vinci needed just over an hour to defeat fellow Italian Camila Giorgi 6-4 6-2 and reach the last eight at Flushing Meadows for the second year in a row.
"Of course I was lucky," said Vinci after scampering off the courts before the clouds burst.
Slovakia's Daniela Hantuchova was closing in on her first quarter-final appearance at the U.S. Open in more than a decade, leading American wildcard Alison Riske 6-3 4-4 at Arthur Ashe Stadium.
Romania's Simona Halep, one of the hottest players on tour with four WTA titles since June, was just one point away from levelling her match with Flavia Pennetta at Louis Armstrong Stadium.
Halep lost the opening set 6-2 and fell 4-2 behind in the second before reeling off three games in a row and reaching set point on her own serve.
Better known as a doubles player, Vinci made light work of her match with Giorgi, who had come through the qualifiers just to get into the main draw.
Giorgi upset former world number one Caroline Wozniacki to reach the fourth round but was outgunned by the more experienced Vinci, who is ranked number one in the world in doubles and is defending her U.S. Open doubles title she won last year with Sara Errani.
"It wasn't my day," Giorgi said. "I didn't play good."
New York's fickle weather has been a major talking point at the U.S. Open for years with each of the last five men's finals spilling into a third week because of rain delays.
For years, U.S. Tennis Association officials balked at the idea of building a roof because of the enormous cost of covering Arthur Ashe Stadium, the largest tennis stadium in the world.
But they have finally relented, announcing two weeks ago that they would commence a massive renovation program, which would include a roof, by 2016 at the earliest. (Editing by Gene Cherry)