Scrum-half Ellis starts Saturday’s fixture, with Croft included among the replacements after playing the full 80 minutes in the win over the Sharks in midweek.
Tour manager Gerald Davies said: "As a tour party we are delighted that we have reached Cape Town and the final Saturday provincial match prior to the Test series with an unbeaten record."
After naming the Lions team, head coach Ian McGeechan said: "The squad is very close as a group of players but now there is now a competitive edge developing as we approach the Test series. That is how it should be.
"No starting players are backing-up from the Sharks match as we continue our desire to see all players involved in at least two full matches prior to Test selection.”
Openside flanker Martyn Williams returns after a shoulder injury and centre Riki Flutey gets his first start of the tour.
Sky Sports are showing live coverage of the Lions series and each match will be shown live on the big screen in the ABC Bar at Welford Road.
British & Irish Lions (v Western Province, Saturday 2pm)
15 Rob Kearney (Leinster/Ireland)
14 Tommy Bowe (Ospreys/Ireland)
13 Keith Earls (Munster/Ireland)
12 Riki Flutey (London Wasps/England)
11 Ugo Monye (Harlequins/England)
10 Stephen Jones (Scarlets/Wales)
9 Harry Ellis (Leicester Tigers/England)
1 Andrew Sheridan (Sale Sharks/England)
2 Matthew Rees (Scarlets/Wales)
3 Phil Vickery (London Wasps/England) captain
4 Donncha O'Callaghan (Munster/Ireland)
5 Nathan Hines (Perpignan/Scotland)
6 Joe Worsley (London Wasps/England)
7 Martyn Williams (Cardiff Blues/Wales)
8 Andy Powell (Cardiff Blues/Wales)
16 Ross Ford (Edinburgh/Scotland)
17 Euan Murray (Northampton/Scotland)
18 Simon Shaw (London Wasps/England)
19 Tom Croft (Leicester Tigers/England)
20 Mike Blair (Edinburgh/Scotland)
21 James Hook (Ospreys/Wales)
22 Gordon D'Arcy (Leinster/Ireland)