NEW YORK, Aug 30 (Reuters) - Fans attending the U.S. Open on Monday confronted long lines at the main gate"s security checkpoint, forcing some ticket holders to wait in the heat for up to two hours to gain entry. The United States Tennis Association, which oversees the event, acknowledged the delays but said the last-minute decision to impose a vaccine requirement for fans was not to blame. The tennis major, which took place amid empty stands last year due to the coronavirus pandemic, commenced on Monday with thousands of visitors again thronging the grounds in New York City"s Queens borough. Citing concerns about the highly contagious Delta variant of the virus, Mayor Bill de Blasio"s office said on Friday that all fans entering Arthur Ashe Stadium would have to show proof of at least one vaccine shot. Tennis officials then extended the mandate to anyone attending the tournament. read more Outside the main entrance at midday, staff members struggled to direct a sea of people awaiting admission. "I"ve been coming since 1963, and this is my last time ever," Howard Lefkin, 71, said, explaining that he had spent two hours trying to get into the grounds. "This is hideous." A handful of fans complained to police officers or shouted at staff, vowing to seek refunds. Others headed around the stadium in search of shorter lines. "If I knew it would be two hours in line, I would have stayed home," said Michael Berman, a 55-year-old attorney from Fair Lawn, New Jersey. A USTA spokesman, Chris Widmaier, said in a statement that the organization was seeking ways to speed up the process. Widmaier cited several factors for the delays, including more later-arriving fans than usual and an "inordinate" number of bags that must be searched by hand. "The process to check proof of vaccination seems to be working smoothly and is not a major contributor hampering entrance to the site," he said. Numerous fans, however, said the line for people with bags actually appeared to be moving much faster than the one for those without. Some said they were concerned that the tightly packed crowd could facilitate COVID-19 spread, even among vaccinated individuals. "The true irony is this is all to keep people safe: they"re checking vaccine cards, but they"re packing everyone in," said Steve Marder, 42, who has attended the Open more than a dozen times. That said, several fans said they appreciated that the requirement added an extra layer of safety. And despite the long wait for some, plenty of tennis aficionados voiced joy at returning to a quintessential New York event. "When we got our tickets, I just about cried just thinking about being back," said Sandra Unger, who traveled from Minneapolis to New York to attend the Open with her brother, Craig. "But I feel outside, I feel safe and the vaccine thing was huge to feel like it"s okay to be here." Stefanie Ebo, 25, attended as a kid and later worked at the Open as a players" assistant for a few years. "Honestly, for me growing up, this was the start of the school year – but also an end-of-summer treat," she said. "I feel like people are even more excited than usual."