Diana Taurasi added 15 points, hitting a 3-pointer with 6:46 left in the third quarter to move into second place on the WNBA's career scoring list.
Taurasi, who now has 6,457 points, reached that total in her 316th game - 166 fewer than Smith, who has 6,452 points, played. Tina Thompson tops the list with 7,488 points.
''It's pretty cool to pass Katie, who obviously is one of the greatest to touch a basketball in our generation,'' Taurasi said. ''That's pretty special to share that list with her and obviously Tina is going to be pretty much impossible to catch.''
Dupree made two free throws in the first quarter to top 4,000 career points and is 22nd on the career scoring list.
''She has the quietest 26 and 14 ever,'' Taurasi said of Dupree. ''She'll make 10 in a row and have the same face when she woke up in the morning. She's unbeliveable when she gets it going. There is no one as smooth as her on the court.''
Dupree took advantage of the Sky's strategy of keying on phenom Brittney Griner, who set a league mark with 11 blocked shots on Sunday against Tulsa. Griner was held to four points on Wednesday but grabbed 11 rebounds and had three blocks for Phoenix (12-3).
''Teams are going to focus a lot on BG (Griner), focus a lot on Diana, so I get quite a few open shots that I have to knock down,'' Dupree said. ''Tonight was my night. I was able to make a lot of shots, get a lot of rebounds but next game, Sunday, it could be somebody different.''
Chicago coach Pokey Charman lamented the strategy.
''It's one of those things where you take Griner away, you can't let Dupree take those points that you take away from (Griner),'' Chatman said. ''It came with a high cost. She is doing a good job of knowing that if I am throwing a double team that somebody else is open, so I credit her for that.''
Allie Quigley scored 16 points to lead the Sky (8-9), who have lost four of six. Epiphanny Prince and Jamierra Faulkner added 11 each.
The league-best Mercury, winners of eight of their last nine, scored 13 consecutive points in the third quarter to stretch their lead to 19.
''That's the speed that we want to play at,'' Phoenix coach Sandy Brondello said. ''But when we play good defense, like we did then, it creates our offense, so I was happy with the way that we finished the game in the second half.''
Faulkner was coming off a 27-point effort in a win against the Sparks on Tuesday, but shot just 4 for 16 from the field against Phoenix.
The Sky again played without leading scorer Elena Delle Donne, who is out indefinitely with Lyme disease, and Courtney Vandersloot, who led the league in assists before spraining her knee. Vandersloot is expected to be out at least six weeks.
''That's what I said, I wanted to keep the tempo high,'' Brondello said of taking advantage of a fatigued Chicago squad. ''So I think in the first half we were playing too slow. We have to push them. They are a tired team. We have to take whatever advantage we can get.''
Penny Taylor added 14 points for Phoenix, which has set a franchise mark for its best record after 15 games.
Dupree had 10 points and nine rebounds in the first half as Phoenix took a 13-point lead. However, Prince hit three 3-pointers in the second quarter to keep Chicago close, pulling to 44-38 at the break.
Phoenix hit 20 of 24 free throws while Chicago was 5 for 8.