The 21-year-old singer was caught wiping away tears during the band's performance in Indonesia on Wednesday, the same day his 22-year-old bandmate officially announced he was quitting the British boy band.
Liam Payne also admitted to a stressful day on his own Twitter, writing, "So glad to be in bed after a long and strange 24 hours."
Meanwhile, Louis Tomlinson took a more positive approach to the news.
"Been a crazy couple of days but know that we are going to work harder than ever to deliver the best album we've ever made for you guys!" he tweeted. "Your support has been incredible, truly incredible so thank you so much!"
"Been a mad few days and your support has been incredible as per usual!" Niall Horan echoed on Thursday. "This in turn spurs us on to make the best music we possibly can. ... You are the best fans in the world and you deserve nothing less from us!"
Not surprisingly, Simon Cowell -- who formed the British boy band during a season of X Factor UK in 2010, when Zayn already showed signs of being uncomfortable with the spotlight -- took the most aggressive approach.
"Sometimes strength is not defined by numbers," he tweeted on Wednesday, obviously referring to the band now continuing as a four-piece. "As always you the fans have shown your amazing support to all the boys. When you have great talent great things happen."
As always you the fans have shown your amazing support to all the boys. When you have great talent great things happen.
In the band's official statement, the remaining 1D members said they respected Zayn's decision and that they would "always be friends."
"We're really sad to see Zayn go, but we totally respect his decision and send him all our love for the future," they said. "The past five years have been beyond amazing, we've gone through so much together, so we will always be friends."
Zayn called his four bandmates "friends for life.”
"I know I have four friends for life in Louis, Liam, Harry and Niall," he said. "I know they will continue to be the best band in the world."