Kyrgios, who won the Japan Open title last week, could face punishment after he was booed off the court during a petulant display.
Footage of Kyrgios’ behaviour went viral online and across social media, with one Australian bookmaker offering to refund losing bets.
“It was just tough. Obviously I played a lot of matches in a row. Physically tired, mentally tired,” the 21-year-old said.
“That’s why I’m trying to work on being able to be consistent every week. Just took the easy way out tonight and obviously didn’t show up at all.”
Kyrgios was hit with a code violation for swearing in the first set and was then booed by the crowd as he walked off the court after failing to face a return from Zverev following his own tame serve to trail 4-1.
Sitting in his chair, the world No 14 became involved in a heated exchange with umpire Ali Nili, who told the Kyrgios to “act professional and play with your best effort the whole time”.
In the second set, when facing game point at 1-1, Kyrgios started to walk off before Zverev’s serve had landed in his side of the court. Later, when Zverev served at 5-1, the Australian responded to heckling from the crowd before conceding the match when he chipped up a return which allowed the German to put away match point after 48 minutes.
Kyrgios defended his exchange with the crowd, saying: “They don’t know what I’m going through, so no, I don’t understand it.
“I don’t owe them anything. It’s my choice. If you don’t like it, I didn’t ask you to come watch. Just leave.
“If you’re so good at giving advice and so good at tennis, why aren’t you as good as me? Why aren’t you on the tour?
“You want to buy a ticket? Come watch me. You know I’m unpredictable. It’s your choice. I don’t owe you anything. Doesn’t affect how I sleep at night.”
Kyrgios was more apologetic when he later wrote on Twitter: “Not good enough today on many levels, I’m better than that. I can go on about excuses but there are none. Sorry #StillAWorkInProgress”.
In yesterday’s other matches Andy Murray, the No 2 seed, coasted through to the last 16 with a 6-3, 6-2 win over American Steve Johnson. No 3 seed Stan Wawrinka saw off Britain’s Kyle Edmund 6-3, 6-4.