Third round leaderboard (GB & Ire unless stated, par 71):
-11 M Siem (Ger) -8 G Fernandez-Castano (Sp), T Bjorn (Den) -7 S Dyson, S Gallacher, G McDowell -6 J Kingston (SA), S Hansen (Den), M Lafeber (Ned), R McEvoy
Siem has had a long wait since his last tournament win
Germany's Marcel Siem produced a bogey-free round of 66 to end the day on 11 under, three shots clear at the end of the third round of the Wales Open.
Siem, whose only previous European Tour win was in 2004, leads Thomas Bjorn and Gonzalo Fernandez-Castano, but the real drama was in the trio one behind them.
Stephen Gallacher's course-record round of 63 put him on seven under, with Graeme McDowell joining him after a 64.
Fellow Ryder Cup hopeful Simon Dyson completed the trio with his own 64.
But Colin Montgomerie, who will captain the European Ryder Cup team at this venue in the autumn, had a day to forget, as a round of 76 saw him plummet from 14th to 47th.
Siem, 29, who was the joint second-round leader, enjoyed a flawless round as he closed in on a first prize of £300,000.
The world number 294 birdied the second, fifth, 11th, 14th and 15th to increase the likelihood of a third successive long-shot victor in the tournament.
Last year the winner was world number 377 Jeppe Huldahl, with world number 164 Scott Strange securing victory 12 months prior to that.
Afterwards the German said that keeping his fiery temper in check was having a positive impact on his game.
"I want to be a better golfer and the only way to do it is to keep my temper," he said after a second successive flawless card.
"I am not so bad now. I just whistle my girlfriend's favourite tune and think about my two little dogs if I feel angry.
"But I've been much better since 2007 when I was trying to win at home in Cologne. I snapped my club in two pieces and when I turned round I saw these kids - I'd scared them. That told me I had to mend my ways."