Painter is back from a calf injury, while Sleightholme has recovered from an ankle problem, with both players able to start against Saracens Storm at Allianz Park.
The Wanderers have made a total of six changes to their line-up as they head into their fourth successive semi-final appearance.
James Grayson captains the side from fly-half for the fourth straight game, while Charlie Davies is back at scrum-half.
Will Davis joins Painter in the front row, and soon-to-be Saint Tui Uru starts in the No.8 shirt.
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Bedford Blues lock Will Carrick-Smith completes the sextet of changes to the Wanderers side as he makes a guest appearance in black, green and gold to complete the tight five.
There are a total of seven guest players in the Wanderers squad.
As for Saracens, they name two future Saints players on their bench, with scrum-half Henry Taylor and centre Reuben Bird-Tulloch both among the replacements.
Ex-Saints Academy wing Rotimi Segun is the only change to the back-line which helped secure a home semi-final with an emphatic victory at Exeter Braves last time out.
Tom Whiteley scored a hat-trick at Sandy Park and continues at nine, while Max Malins, who also grabbed a try, starts at fly-half.
In the pack, Nick Isiekwe switches to second row to accommodate the returning Sean Reffell, who partners Ben Earl and Sione Vailanu in the back row.
Entry to tonight's game at Allianz Park is free to all supporters.
Saracens Storm: Gallagher; Obatoyinbo, Morris, Griffiths, Segun; Malins, Whiteley; Adams-Hale, Woolstencroft, Judge; Day, Isiekwe; Reffell, Earl, Vailanu.
Replacements: Tolofua, Thompson-Stringer, Figallo, Kpoku, Maka, Taylor, Watson, Bird-Tulloch.
Wanderers: Emery; Sleightholme, Kellaway, Strachan, Pisi; Grayson (c), Davies; Davis, Newman (guest), Painter; Jubb (guest), Carrick-Smith (guest); Coles, Onojaife, Uru.
Replacements: Taylor (guest), Trinder, Ford-Robinson, Betteridge (guest), Laxton (guest), Tupai, Freeman (Under-18s), Worley.