An emotional Mark Williams has quit as coach of AFL club Port Adelaide, saying his position had become untenable.

After seven straight losses Williams will coach the Power for the last time on Friday night against Collingwood.

"It's a great journey, but it comes to an end right now," he told a media conference at the club's headquarters.

"For the good of the club, that's paramount.

After thanking his family and all the club's staff and supporters, Williams elected to skip several pages of a prepared statement.

However, he did try to lighten the mood with a joke, telling journalists he was leaving because of illness and fatigue.

"The supporters have become sick and tired of me," he said.

"That's humour, what do you think?"

Port Adelaide chairman Brett Duncanson said the decision for Williams to leave was one mutually agreed on.

He said Williams had always put others before himself and his players had all felt the benefit of his compassionate and caring legacy.

"Simply, Choc has put the club before himself, as he always has," Duncanson said.

Williams has coached the Power since 1999 and leaves after more than 11 seasons and 274 games in charge.

His fate was probably sealed with the club's seventh straight loss last weekend, Port's longest losing streak in its AFL history.

The 51-year-old delivered Port a premiership in 2004 but the Power has missed the finals in three of the past four seasons.

Speculation has linked Williams to a role at Greater Western Sydney as an assistant to head coach Kevin Sheedy when the new AFL club enters the competition in 2012.

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