Local favorite Kaymer had three straight bogeys and a double bogey in a 1-over 73 that included four birdies. He finished on level-par 144 overall.
''[This] was a big step for me to accept that you have golf courses that don't suit your play. so it's OK for me,'' said Kaymer, who was born in nearby Duesseldorf.
''It is how it is. I did my very best and level par was the best I could do.''
Former champion Pablo Larrazabal made nine birdies - including five in a row - for a 9-under 63, matching the best round so far. The Spaniard is on 12 under overall with overnight leaders Danny Willett of England and Rafa Cabrera-Bello of Spain.
''I played great today, I played great yesterday,'' said Larrazabal, who won the tournament in 2011.
Emiliano Grillo of Argentina later carded his second successive 66 to join the leaders.
Cabrera-Bello double hit for his first missed shot of the week on the par-5 seventh but made five birdies while Willett, the 2012 tournament winner, stayed bogey-free to match his Spanish playing partner's 4-under 68.
''I was a little unlucky coming down the last few holes but, overall, pleased with the minus-4,'' Cabrera-Bello said.
Andy Sullivan was level with the leaders with seven holes to play when play was suspended for almost 90 minutes because of a heavy storm. The Englishman dropped a shot on the par-5 No. 3 once play resumed and finished with a double bogey to finish on 9 under, three off the pace.
Spain's Miguel Angel Jimenez, Scotland's Craig Lee, South Africa's Branden Grace, Germany's Alex Cejka, Denmark's Andreas Harto and Sweden's Robert Karlsson were tied in fifth place, two shots behind the leaders. Jimenez won the tournament in 2004.
Paul Waring made seven birdies and an eagle to complete a 63. The Englishman was in an eight-way tie for 11th on 9 under.
Second-ranked Henrik Stenson, who won in 2006, again finished the round 4 under with four birdies. The Swede is among a group of 10 players sharing 19th place including two-time winner Thomas Bjorn of Denmark on 8 under overall.
Former winner John Daly of the United States shot 67 and is tied for 29th on 7 under.