This, despite a remarkable escape from an unsavoury hazard by the defending champion.
Colsaerts beat McDowell in the final last year and also knocked the Northern Irishman out in the quarter-finals in 2011, but McDowell made it third time lucky with a 2&1 win in the quarter-finals at Thracian Cliffs.
Three up after nine, McDowell seemed set to go further ahead when Colsaerts' drive on the short par-four 10th finished inside a public toilet.
But after taking a penalty drop - bringing new meaning to the phrase 'nearest point of relief' - Colsaerts pitched onto the green and holed from 15ft for par to halve the hole.
Colsaerts could be heard laughing as he pleaded with the referee not to make him drop the ball inside a bathroom on live television, but had to drop it twice as it rolled nearer the hole the first time.
"I was laughing because I was going to look like a clown," Colsaerts said.
"I'm going to make 'A Question of Sport' and I'm sure it's going to make the news, some Belgian dude dropping the ball in a toilet. It's fun to be recognised for stupid stuff like that."
After escaping with his par, Colsaerts got back into the match with a birdie on the 13th - where McDowell was going to drop at least one shot - and by winning the 14th with a par.
However, he missed a birdie putt from seven feet on the 16th to get back on level terms and then hit two clumsy pitches on the par-three 17th to allow McDowell to close out the win.
McDowell will face Branden Grace in the last four after he beat England's Chris Wood 2&1 in the quarter-finals, while the other semi-final sees another South African Thomas Aiken take on Thongchai Jaidee, two men who beat Ryder Cup star Ian Poulter in the group stages.
Aiken beat Italy's Francesco Molinari 3&2 while Jaidee defeated Scotland's Scott Jamieson 4&3.
McDowell, who almost squandered a four-hole lead with six to play before beating Bo Van Pelt on the 18th in the last 16 this morning, said: "I keep giving guys a look in and you don't want to give them too many chances.
"I was looking like going four up on the 10th and Nicolas then literally makes par from the toilet. It's when the Rules of Golf get a bit funny and wacky and people watching will be wondering what's going on, but what a four he made.
"I'm happy to get through and excited to be in the semi-finals. It's a very historic event and a trophy with a lot of legendary names on it. The golden years at Wentworth I remember watching a lot of my heroes win it and I would dearly love to add my name to the list.
"Branden will be a very tough competitor. My game plan has been to hit fairways and greens and not give my opponent too much of an opportunity and it has worked pretty well so far."
McDowell insisted that his energy levels were high despite playing 35 holes today, but Grace and Aiken both admitted to be struggling to cope with the undulating course, despite players being driven in buggies between many of the greens and tees.
"Thirty six holes a day is definitely starting to take its physical toll," said Aiken, who played both his group matches on Friday as one of the eight unseeded players and needed 19 holes to defeat Ireland's Shane Lowry in the last 16.
"I wish I was a little bit fitter, but I think if you can get through the match tomorrow morning you'll be extremely focused for the afternoon."
Grace, who ended Brett Rumford's bid to join Nick Faldo and Seve Ballesteros in winning three European Tour events in a row with a 4&3 win in the last 16, turns 25 on Monday but added: "I have little aches and pains, a little stiff.
"It's been a long day and I can't remember the last time I played 36 holes in a day."