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One fan hacked 50,000 printers over the weekend to print out a message urging people to subscribe to his channel.
Both channels have more than 73 million subscribers, with PewDiePie currently in the lead by about 300,000.
T-Series shares Bollywood film trailers and music videos, while PewDiePie posts video game commentaries and vlogs.
The anonymous printer hacker, going by the name Hacker Giraffe, said they had identified 800,000 printers with open security settings and selected 50,000 to print out support for PewDiePie.
The vulnerability could have been used to cause "major havoc", the hacker told technology news website The Verge.
"Hackers could have stolen files, installed malware, caused physical damage to the printers and even used the printer as a foothold into the inner network," they said.
"The most horrifying part is I never considered hacking printers before, the whole learning, downloading and scripting process took no more than 30 minutes."
Other YouTubers, including Mr Beast and Markiplier, who between them have 33 million subscribers, have also made videos urging their followers to subscribe to PewDiePie.
T-Series posts Bollywood movie trailers and music videos.
In a video he posted himself about the fight to retain his crown, PewDiePie - whose real name is Felix Kjellberg - said he was enjoying the support.
"All of this support to keep me on top is so funny. I love it. Please keep it up," he said.
"But don't do anything illegal OK... because that will look bad on me."
According to the website Social Blade, PewDiePie is gaining an average of 144,656 subscribers per day while T-Series is averaging 157,656.
T-Series has not posted anything about the situation but it has been described by some observers as a David versus Goliath battle.
"T-Series, having started in the 1980s, is a legacy brand in India, a country with a population of 1.3 billion people," wrote Matt Binder on the tech site Mashable.
"It has employees and an entire roster full of musical artists and Bollywood superstars that it relies on to pump out as many as half a dozen highly produced music videos per day.
"In contrast, PewDiePie is just one guy putting out one vlog or video game stream per day."
PewDiePie controversy
PewDiePie has faced criticism in the past over some of his YouTube content.
In 2017, Disney cut ties with him after several videos he released were found to contain Nazi references or anti-Semitic imagery.
Later in the year, he apologised for using a racist slur during a video live stream.
In the summer of 2018, he deleted a post that mocked singer Demi Lovato, who was in hospital being treated for a drug overdose.
It showed Demi asking her mum for money for a burger before spending it on heroin instead.
Afterwards he said he "didn't fully know about the situation" and realised his post had been "insensitive".
