A chiseled-looking Kitajima, who won the 100 and 200 breaststroke double at the 2004 and 2008 Olympics, clocked the fastest time of 1:00.23 minutes at Tokyo's Tatsumi Pool.
"These were the prelims so my body felt light," Kitajima told reporters after beating Ryo Tateishi by 0.14 seconds in the national meet, which doubles at Japan's Olympic qualifiers.
"Ryo was swimming within himself too. We're both in good shape and I want to take him on in the (Tuesday's) final and win an Olympic spot."
"My body feels light now, which I'm pleased about," added the 29-year-old Kitajima, who was competing for the first time since the World Cup in November.
"But I also have to think about the 200 metres too."