Nadal was serving for the second set at 5-4 when a woman yelled out and made hand gestures as he prepared to serve. She yelled again at 30-0 and stadium security escorted her out of Rod Laver Arena. The rest of the crowd started booing and Nadal laughed.
He went back to work after the brief delay and hit two aces to finish off the set.
The 20-time major champion completed the match against the 177th-ranked Mmoh in 1 hour, 47 minutes and said it was a "positive evening for me."
Except for one thing. He was asked if he knew the woman who caused the delay. He said he didn't and added, "Honestly, I don't want to know."
More than 21,000 fans attended on Day 4. The Australian Open is the first major in a year to allow sizeable crowds with the government allowing up to 50% capacity. No fans were allowed at the US Open because of the COVID-19 pandemic, and the number of spectators at the French Open was extremely limited.
The day after Nick Kyrgios sent John Cain Arena into delirium with a thrilling comeback, 267th-ranked Greek-Australian Kokkinakis appeared set for the boilover of the tournament when he threaded a backhand down the line to send the match into a fifth at Rod Laver Arena.
It was not to be, though, as Tsitsipas captured the decisive break in the fifth game and held on grimly to close out an epic contest which lasted four hours and 32 minutes.
"Thanasi is a great competitor and a great fighter. As you saw it was very difficult facing him today," Tsitsipas said on court postmatch."I just want to go for a nice bath right now, that's all I'm thinking.
He added: "Despite the difficulty of the match today I really enjoyed being out here and competing and showing the world a really high quality of tennis."
Tsitsipas has a big following within Melbourne's large Greek community, but he was playing an Aussie for the first time at the Australian Open and had to contend with having a share support in the crowd.
The No. 5 seed will meet Sweden's Mikael Ymer for a place in the fourth round.
Kokkinakis didn't play at all in 2020 because of illness and injuries and this is his first major tournament since the 2019 US Open. His first-round win over over Kwon Soon-woo was his first in the Australian Open main draw since 2015.
With no apparent sponsorship from apparel giants, Kokkinakis caused a stir when he wore a black, $4.65 Kmart T-shirt from a local shopping mall in his first-round match.
Against Tsitsipas, he wore a crisp white T-shirt but had a small logo stitched onto the sleeves, having picked up an endorsement deal from a workplace safety firm.
The company got its money's worth as Kokkinakis took the first set, with Tsitsipas serving up a double fault on set point.
Tsitsipas knuckled down in the next two sets and was cruising to victory with a break in the fourth before Kokkinakis rallied and closed out a thrilling tiebreak with an exquisite backhand passing shot.
World No. 65 Lopez is playing a record-extending 75th consecutive Grand Slam event, having not missed any since the French Open in 2002.
Only Roger Federer, who has skipped this year's Australian Open due to an injury, has played more Slams overall, with 79 compared to Lopez's tally of 76.
Mackenzie McDonald is also back in the third round of a Grand Slam tournament for the first time since a torn hamstring tendon left him unable to walk for weeks. The American eliminated No. 22 seed Borna Coric 6-4, 6-2, 4-6, 6-4.
Information from Reuters and Associated Press contributed to this report.