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Phillips, who is the most capped No 9 in Welsh history, butted heads with Marshall during their careers before the pair eventually became teammates at Ospreys in 2007.
Marshall previously revealed that the two were involved in a number of verbal spats, concluding that Phillips didn't like him.
However, Phillips said their rivalry "wasn't like that at all".
"The way I looked at it was: He's Justin Marshall, he's got 80 caps, he's been one of the best in the world and I'm going to try and beat him. I want to be the best in the world," Phillips told Wales Online.
"He's arguably one of the best that's played the game and you want to push yourself to be as good as you can be.
"It was to spark that competitiveness in me or maybe, at times, I was a bit nervous or edgy.
"It was never personal with anyone. You just want to win so much you use anything you can."
Despite Phillips' insistence that there was no bad blood between them, he said they weren't exactly close either when they became teammates.
"I didn't really speak to him that often but what I did see was that he was an absolute winner - a competitor," said Phillips.
"Even though he was at the end of his career, he wanted to start every game. He wasn't always great in training on a Tuesday or Wednesday, maybe a bit stiff, but come Saturday he was easily one of the best players on the field.
"That's what I took from him really. Tuesday is important but it's all about Saturday. That's when it matters.
"I remember I got picked against Gloucester in the Heineken Cup. As soon as it was announced, he threw his toys out of the pram a little bit and that surprised me. But the era that he came through was 'dog eat dog' and that's what made him so good.
"I never saw myself as competing against him because we were at different stages in our careers. But what blew me away was just how much he still wanted to win and how good he was on game day."
Phillips, who recently joined sports management company World in Motion as a rugby consultant, also spoke about his interactions with former All Blacks first-five Dan Carter, who he says was excellent both on and off the field.
"He was exceptional," said Phillips. "He never made a mistake and always made the game look effortless.
"He'd nail every kick from the touchline. At times he didn't even look bothered and suddenly he'd do something extraordinary."
Phillips did, however, send a shot at the All Blacks, saying the three time World Cup winners would often get favourable calls from referees.
"I remember playing in the Paris derby and I was chasing him (Carter) down the touchline and I could see his foot went into touch so I stopped," Phillips said.
"But the touch judge didn't put his flag up. So I've stopped and had a go at the touch judge: 'He was in touch, mun!'
"The flag didn't go up just because it was Dan Carter, it did my head in! I used to think they [All Blacks] used to get a lot of decisions.
"But he was great to play with, comfortable on the ball and very cool in every situation. He was class."