The pony-tailed Spaniard stole a march on the youngsters after mixing two birdies and two bogeys in a level-par 71 that gave him a three-under tally of 139.
"Being on top of this major championship field makes you feel a lot of pressure but that's what I like to feel," he told reporters on another glorious day of sunshine at Muirfield.
"I like to feel the pressure. As long as you can handle that, it's no problem. I feel comfortable."
Jimenez said he always felt relaxed because he had a job that he adored.
"Enjoy yourself in what you do in life," added the cigar-smoking, Rioja-drinking Spaniard. "That's the secret and that's what I'm doing.
"I love what I'm doing. I play golf, I do this for a living and I've kept doing the same thing for 25 years.
"Sometimes maybe you will say to me, 'That's too many years' but you're wrong. It's the only thing I like to do in my life.
"This is fun to me," added Jimenez. "Having fun does not mean you have the biggest smile and you fall on the floor laughing - fun is when you enjoy what you're doing."
Jimenez has still not fully recovered from the broken leg he sustained in a skiing accident in December and is also suffering from tennis elbow.
Yet the former Ryder Cup player believes he is capable of becoming the oldest major winner of all time, eclipsing American Julius Boros who was 48 when he triumphed at the 1968 U.S. PGA Championship.
"I keep elastic and flexible," said Jimenez. "I'm still training and walking and still able to shoot low.
"If I cannot shoot low then I will not be here anymore, don't worry about that. I wouldn't waste my time travelling around the world kicking my backside," he said to howls of laughter.
Asked how he was leading the British Open at the age of 49, he replied: "Why? Have I not the right to do it? Is it only the young people who can do that?
"I've been on the tour 25 years, I have 19 victories, and I would love to have a first major in my career. Why not this one?"
Jimenez warms up for half an hour before he takes to the course and not for him is the lifting weights regime that many of the young generation of golfers adopt.
"You have to do the same things you do every day, you don't need to change anything," he said. "As soon as I finish here I'll go and be with my girlfriend, my sons, and we will have dinner like I do every day.
"I don't need to do anything special because I'm leading and I won't be going to bed at 10 o'clock... I'll go when I feel like it... after I smoke my cigar," he said with a big grin.
(Editing by Toby Davis)
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