Phillips rejected an invitation to play against the Springboks in a bid to land a blow in his battle with Marshall for the number nine jersey.
'I got the invite a few months ago and I was delighted,' said the Wales half-back.
'But it's my first season with the Ospreys and I want to secure the number one spot.
'Justin is a great scrum-half so there is good competition for me at the region. We are both fighting hard for the jersey.
'I want to play as many times as possible for the Ospreys so I'm delighted to be starting for them.'
The clash at Twickenham Stoop is the only meaningful fixture in Sunday's final round of pool games.
The Ospreys need just one match point or must prevent Harlequins picking up a maximum five-point haul to book their place in Sunday's semi-final draw.
Quins make two changes to the side edged 27-25 at Gloucester in the Premiership last weekend.
Adrian Jarvis replaces the injured Chris Malone at fly-half and Will Skinner comes in for Paul Volley at openside.
England winger David Strettle continues his midfield partnership with former Springbok De Wet Barry, with Simon Keogh and Ugo Monye on the wings.
Bristol are missing England hooker Mark Regan for their dead rubber against Leeds.
Regan has defied club orders to turn out for the Barbarians and could face disciplinary action next week.