Littler is out to keep hold of the title, which was his first on the PDC circuit, in his first tournament just a fortnight after his triumph at Alexandra Palace in London.
The 17-year-old was mostly in control of the match as Ilagan threatened, but a big 142 checkout in the eighth leg helped him on his way to the quarter-finals where he will face Gerwyn Price, who overcame Lok Yin Lee 6-3.
"It felt good and I'm just happy to get past that first game," Littler said.
"I played alright in patches and did the right things at the right time.
"It would mean the world to come back and retain this title. I know it will be a long night tomorrow but I'll be up for it."
There was a shock earlier in the day when the tournament's opening game saw former world champion Rob Cross crash out after a shock 6-3 defeat to Paolo Nebrida.
Nebrida wrote his name into the history books by becoming the first player from the Philippines to win a match on the World Series.
World number one Luke Humphries made light work of his first-round match by thrashing local player Abdulla Saeed 6-0 and entertained the crowd with several ton-plus finishes.
Stephen Bunting saw off Nitin Kumar 6-2 to set up a clash against Chris Dobey, who easily dispatched Basem Mahmood 6-0.
Peter Wright got past Alexis Toylo 6-3 before Nathan Aspinall ended the session with a 6-2 win over China's Xiaochen Zong.
The quarter-finals, semi-finals and final take place later on Friday.
Bahrain Darts Masters first round results:
Stephen Bunting 6-2 Nitin Kumar
Luke Humphries 6-0 Abdulla Saeed