The 47-year-old from Santerno pipped Great Britain's Rachel Cawthorn into first place in the K1, raising the prospect of a gold medal on Thursday.
London 2012 is the eighth Games for Idem, who first won an Olympic bronze medal in Los Angeles in 1984 when she was only 19.
Since then, she has won bronze in Atlanta, gold in Sydney and silver at both Athens and Beijing.
If she wins gold again, she will not break the record for the oldest Olympian to win a gold medal as Oscar Swahn became a champion for Shooting at the age of 64 and 280 days at the 1912 Games, while Sybil Newall was 53 and 275 days old when she won in Archery in 1908.
Speaking to reporters after qualifying for the final of the K1, Idem said she still trains for four hours a day.
She said: 'Every day you have to work on your motivation. You always have to think about it.
'You go out really tired, you go out with bad weather, you go out with problems at home. But you have to go. This is it - the effort you have to put in if you want to succeed.
'I am very lucky. Many people do the same amount of work as I do and they don't go to the Olympics, so my motivation comes from the fact that I was lucky and I was good all these years.'
They finished 40 seconds behind the winning Chinese pair of Maoxing Huang and Qiang Li in the men's Canoe Double (C2) 1000m after they were stranded on the start line when their boat was caught up in the wash of their competitors.
As the boat started rocking, they were unable to move for fear of capsizing and had to wait until the water had calmed.
The Australians also had a better day on the water, with the men's Kayak Four (K4) 1000m team of Dave Smith, Tate Smith, Murray Stewart and Jacob Clear winning their semi-final comfortably.