"It was a good round today but a disappointing one, but how many people can get disappointed with nine under?" the 25-year-old Lewis told reporters at Himmerland Golf & Spa Resort.
"It's a tough one but everyone can tell it (a chance of a 59) was on there and unfortunately I came up just short. I hit some good putts, I hit some bad putts. A couple of good ones that missed, and a couple of bad ones that holed.
"In all it was fine," said Lewis, whose only European Tour victory came at the 2011 Portugal Masters.
The in-form Pieters served a timely reminder to Europe's Ryder Cup captain Darren Clarke that he would be a worthy selection as one of the three wild-card picks to be announced by the Northern Irishman on Aug. 30.
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Europe have already assembled their first nine in a group featuring Rory McIlroy, British Open winner Henrik Stenson, Olympic champion Justin Rose and cup veteran Sergio Garcia, plus five rookies including Masters winner Danny Willett.
"It was stress-free today," Pieters, 24, said after carding 10 birdies and a lone bogey. "I drove it well off the tee, hit it close and made a bunch of putts.
"I did all I could in front of Darren today. He has a lot on his mind other than golf and we will just have to wait and see. If I do win this week, it might get him thinking, but I just have tomorrow to worry about."
A twice champion on the European Tour, Pieters placed fourth at the Rio Olympics earlier this month before finishing second at last week's Czech Masters.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Andrew Both)
