Kahleah Copper scored 20 of her 36 points in the fourth quarter as the Mercury (12-11) trimmed what had been a 31-point third-quarter deficit to single digits.
Mitchell gave Indiana (10-14) an 81-50 lead with 1:55 left in the third quarter before the Mercury scored the next 17 to get with 81-67 on Sophie Cunningham’s 3-pointer. Copper converted a shot in the lane with 5:02 left to get Phoenix to 85-76.
Clark tied the franchise record with her 13th assist on a cross-court pass to Mitchell for a 3-pointer and a 92-83 lead with 1:33 left.
Phoenix’s comeback came without center Brittney Griner, who collided with a teammate in the first half and stayed on the floor before a timeout was called. The team announced Griner wouldn’t return due to a right hip injury. The Mercury were already without starters Diana Taurasi (leg) and Natasha Cloud (knee).
Clark secured the double-double early in the third on a pass to Mitchell for a 62-40 lead. Clark also set a franchise record for assists in a quarter when she had seven in the first.
Aliyah Boston added 21 points and 13 rebounds for Indiana while NaLyssa Smith had 14 points and 10 rebounds.
Cunningham added 21 points for Phoenix, which had won three in a row.
Clark completed a three-point play with eight seconds left in the first half as Indiana closed on a 10-2 run for a 55-35 lead. Mitchell scored 17 points in the first half, Clark had 15 and Boston added 10.
