Petr's other daughter Jessica won golf's Australian Open seven years ago, while son Sebastian captured the nation's 2018 boys' singles tennis title.
"We have something we will be cherishing for the rest of our lives," Petr said in a telephone interview with Reuters.
"Nelly wanted to win, I don't want to say badly, but it means quite a bit to her to complete the Aussie Slam.
"She wanted to be in it, and I think it's great for the family, the grand slam. It's four acts and we were missing the last one. It's very unique, and like blackjack it took 21 years."
Petr had a chance for a quick phone chat with Nelly, before turning his attention to Sebastian.
"In the last 48 hours I've slept probably two hours," he said. "I watched Nelly till the end and then our son was also playing in Turkey (in a futures tour tournament).
"He lost 6-3 in the final set but that's a great result."
The Korda children were blessed with great athletic genes -- mother Regina was an Olympic tennis player -- which the daughters have parlayed into success at the highest level.
Sebastian seemingly is also destined for big things in tennis.
Jessica, 25, has won five times on the LPGA Tour, while 20-year-old Nelly has two victories on the circuit.
Czech-born Petr said he and Regina had never pushed their children to join the tennis ranks.
"We taught them multiple sports and Jessica picked golf and for Nelly it was easier to follow her footsteps.
"I think they have a great rivalry. They push each other on the one hand but on the other hand if one is struggling the other is always there."
Petr now spends more time with Sebastian, at 18 the youngest of the three children, often as his hitting partner.
"When you have a little bird, you're trying to help it fly from the nest," he said.
"I like to help Sebastian transform from junior tennis to men's tennis. He will need, not my attention, but time together."
Petr said all three children had a temper, though nothing out of the ordinary. Despite his sporting pedigree as a grand slam champion, he tries not to push too much unsolicited advice on them.
"Regina and I are here for any question they need," he said. "At end of day, I call it a family recipe."
(Reporting by Andrew Both in Cary, North Carolina; editing by Clare Fallon)
