He was at 10-under 132 overall, one stroke ahead of 2006 champion Bradley Dredge (65) and Miguel Angel Jimenez (68).
Overnight leaders Simon Dyson (71) and Brett Rumford (73) fell back. Dyson shot a 63 in the opening round but dropped to fourth, tied with Paul McGinley (68), Ross McGowan (67) and Angelo Que (65).
Rumford shared eighth place with seven others.
Aiken began his round, the best of the day, with a birdie and followed that with his only bogey. He made five more birdies and an eagle at the 14th.
“I had 242 yards to the pin and crunched a 2-iron to eight feet,” said Aiken, who has finished in the top 10 seven times this year. “It has been a fantastic season and it’ll be more fantastic if I can cap it off with a win.”
The European Masters is the first event in qualifying for next year’s European Ryder Cup team.