The Oban man shot a flawless 65 on Friday to start the third round with a four-shot advantage at 11 under par, but he saw that lead evaporate at the turn as he carded two bogeys while playing partner Ritthammer rolled in two birdies.
The Nuremberg native and MacIntyre bogeyed the tenth to drop to eight under before Ritthammer showed nerves of steel to bounce back and birdie the 11th and 12th to surge two shots ahead.
However, the German let MacIntyre and Ryder Cup star Paul Casey back in at eight under following successive bogeys at the 14th and 15th.
MacIntyre and Ritthammer both birdied the last to move one shot ahead of world No 17 Casey at Green Eagle Golf Courses.
“Things didn’t go my way today,” MacIntyre said after his round of 74.
“I fought well until the end. It doesn’t matter where I am on the leaderboard, bottom or top, every point counts. Today I just showed my fight and to birdie the last was huge to get a tie for the lead.
“It’s just about the belief. I’m going to go out there tomorrow and play my best golf. If it’s enough, it’s enough. If it’s not, it’s not.
“We are always going to learn and today was a huge learning experience.”
Casey, who led after the first round in Hamburg, rolled in five birdies but blotted his card with two bogeys to sit in solo third, while Spaniard Pablo Larrazabal and Austria’s Matthias Schwab are one further back on seven under par.