Kitsos, 55, suffered a massive heart attack while on a United Airlines flight to San Francisco, his family said. He was due to board a connecting flight to Las Vegas, where the tournament is being held.
His wife of 20 years, Stella Spanakos, said Kitsos was a self-made businessman who started a plus-sized clothing line for women and often contributed to charities.
"He was extremely humble," said Spanakos, 53. "He was a very private person. He didn't like show. If he wasn't at work and not at poker, he was home."
She said Kitsos fell in love with the competitive action of the game. "It's that whole big rush," she said.
In his youth, Kitsos, the child of Greek immigrants, attended C.W. Post Campus of Long Island University and played professional basketball in Greece. He later started his clothing line, KB Studio, spent time studying and playing poker, and gave back to causes he believed in, his family said.
One of those causes was autism, his wife said. The couple's son, Nicholas Spanakos-Kitsos, 18, was diagnosed with autism and attends The Elija School in Levittown, a full-day program for autistic students ages 3 to 21. The school's co-founder, Nicole Sugrue, said Kitsos' $20,000 donation helped open the school in 2006.
"Paul's donations were monumental in the founding of the school," she said.
Patrick Bardsley, 22, said the couple took him into their home when he came to Long Island from the United Kingdom to study special education. He had befriended Nicholas at a summer camp upstate.
"He completely welcomed me into his home," Bardsley said. "He always said, be happy and . . . be kind to everyone."
On Monday, Spanakos said, before her husband boarded the flight, he sent a text message to his son: "Daddy misses you already." His son wrote back: "Good luck, you're the best."
Kitsos' family and friends said he had spent three days training for the series online and reading books about poker. He owned more than 100 books on poker, his wife said.
"He was very excited," she said. "He definitely wanted to win a World Series."
In lieu of flowers, the family asks for donations to The Elija School in Levittown.
A wake will be held Thursday and Friday from 3 to 5 p.m. and 7 to 9 p.m. at Fairchild Sons Funeral Home in Manhasset. Religious services will be held there at noon Saturday.