ITALY, 17 Sept 2015, (AFP) - The 2012 Ryder Cup winner, who began his round at the 10th hole, was even par after four holes before beginning his hot streak at 14 with his first gain of the day.
Seven consecutive birdies followed from 15 to 18 before further putts fell at the first, second and third holes only for his amazing run to finally come to an end on the fourth hole where he narrowly missed a 15ft putt.
He becomes just the 11th player on the European Tour to gain shots at eight straight holes which leaves him two strokes clear of a group of three players that includes India's Shiv Kapur, Italian hope Francesco Molinari and Kristoffer Broberg of Sweden.
"It all felt pretty easy to be fair," said Colsaerts who is chasing a third victory on the EPGA Tour.
"We all know what it's like when you get on fire and you keep going. I didn't know how many it was, they were just coming. I stopped (counting) at six and it kept going for a while which was good.
"I was playing very nicely, I only holed a very long one on the second, all the other ones were very makeable."
He added: "It's a combination of a lot of things, I was waiting for a round like this where everything clicks together.
"I keep on missing shots here and there that in the end make a difference, where today everything just went together. I'm working on a lot of things and it seems to pay off," added the 32-year-old who turned pro in 2000 on his 18th birthday.
The second round at the par-72 Golf Club Milano in Monza is set for Friday.
Leading scores after the first round of the Italian Open being held at the par-72 Golf Club Milano on Thursday:
63 - Nicolas Colsaerts (BEL)
65 - Shiv Kapur (IND), Francesco Molinari (ITA), Kristoffer Broberg (SWE)
66 - Fabrizio Zanotti (PAR) Borja Virto (ESP)
68 - Paul Maddy (ENG), Danny Willett (ENG) Mikael Lundberg (SWE) Yang Y.E. (KOR), Jeev Milkha Singh (IND), Florian Fritsch (GER), Paul McGinley (IRL), Anders Hansen (DEN), Ricardo González (ARG), Jason Scrivener (AUS)
72 - Eddie Pepperell (ENG), Richard Finch (ENG) Tom Lewis (ENG), Robert Rock (ENG), Stuart Manley (WAL) Garth Mulroy (RSA) Joon Kim (ITA), Grégory Bourdy (FRA) Damien McGrane (IRL) Ignacio Elvira (ESP), Felipe Aguilar (CHI), Daniel Gaunt (AUS), Marcus Fraser (AUS), Thorbjorn Olesen (DEN)
74 - Richie Ramsay (SCO), Pelle Edberg (SWE) Niccolò Quintarelli (ITA) Daniel Vancsik (ARG) Grégory Havret (FRA)