Jiménez, who finished fourth in the Masters, then won his debut event on the Champions Tour, birdied six of his first 13 holes, but bogeyed two holes down the stretch.
"I'm a little disappointed with that," Jiménez, 50, was quoted by the European Tour website. “I didn't hit it very well the last few holes.
"I played very well the first nine holes and was solid until the 14th but made a bad bogey on the 15th where I three-putted from eight meters and also bogeyed the last."
Pieters, 22, a pro for less than a year, shot 71.
"Playing with Jiménez will be fun, I'll get to learn a lot from him and I'll just soak it all in,” Pieters said. “I've never played with him or any of the big names. The crowd will also be bigger and all the support will go to him but I just have to play my own game."
Jiménez is already the oldest winner in European Tour history, beating his own record by winning the Hong Kong Open for a record fourth time last December at age 49 years and 337 days .
The Spaniard has never won his national Open. "It would be nice,” he said. “I'd love to win the Spanish Open, of course, and we'll see what happens tomorrow."