Kahleah Copper could not be stopped and has the No. 6-seeded Chicago Sky one win away from their first WNBA championship on a clunky night for the No. 5 Phoenix Mercury.

Copper scored 20 points in an efficient first-half outing to lead the Sky to an 86-50 victory in Game 3 at a sold-out Wintrust Arena in Chicago on Friday. It's the largest margin of victory in WNBA Finals history at 36 points.

Game 4 of the best-of-five series is Sunday in Chicago at 3 p.m. ET on ESPN.

The veteran wing couldn't be stopped in the first half. She blew by defenders with her quick first step, finishing tough looks at the basket with and-1s, and stepped back for 3 if a defender dropped back on her. Her 20 first-half points were on 6-for-8 shooting, including making 2-of-3 3-pointers, and hitting all six free-throw attempts. She finished with 22 points in 24 minutes.

Candace Parker had 13 points in 24 minutes with four rebounds and three assists. Courtney Vandersloot had another 10 assists this Finals. It's her 10th career postseason game with double-digit assists, breaking the tie she held with Sue Bird for the record.

The Sky put together unmatched streaks in the second quarter and one of the best defensive outings of the year that resulted in a 46-24 lead at the half. They kept that margin at around 20-24 points throughout the game, prompting the sides to pull their starters early in the fourth quarter.

Phoenix is capable of closing a gap, even when it's 20 points, but could never get its offense in rhythm nor could it stop the Sky's scoring. The Mercury are 3-0 this playoffs after a loss and Diana Taurasi is nearly undefeated in elimination games.

Sky cruise in first half with Kahleah Copper

Copper nearly eclipsed the output of the entire Mercury offense, keying Chicago to a 46-24 lead at the half. Phoenix's 24 points was the club's worse first-half output since 2013, and was half of the first-half offense they'd had throughout their playoff run.

The 22-point halftime difference tied for the largest in WNBA Finals history, per ESPN Stats and Info. It ties the mark set in 2014 by the same teams, except Phoenix led Chicago, 42-20, in Game 1 of that one.

The Mercury couldn't get anything going and didn't have the rims on their side, either. They missed nine consecutive field goals in the first quarter, much of which were clunkers on the unforgiving baskets. They shot 24.2% (8-for-34) in the first half, a mark that at one point in the second quarter stood at 18.2% (4-for-22). While Copper collected 20 points, no one on the Mercury roster scored more than five.

The Sky shot better than 50% and used an 8-0 run over the first-quarter break followed by the start of a 10-0 run a minute later to begin to create distance. The bench played good minutes in the period to give the starters rest and even extend the lead. And the team collectively, led by Copper, was able to get to the free-throw line after taking only four shots there in Game 2.

The Sky were 50% from the floor (30-for-60) and 44.4% from range (8-for-18). In a stark contract from previous games, they took 22 free throws and hit 18. The bench closed it out and Dana Evans, an all-rookie selection out of Louisville, put Sky fans back into deafening roars with three consecutive 3-pointers.

Sky's defense shuts down Mercury offense

Defensively the Sky disrupted everything and collapsed on Brittney Griner enough she didn't have the easy baskets and passes she did on Wednesday. By the time Mercury head coach Sandy Brondello went to the bench at 8:49 of the fourth, Griner had 16 points whereas the rest of the club had 22 combined.

Skylar Diggins-Smith was second with seven points on 2-for-9 shooting with three assists. Taurasi, who ahead of the game talked about her fine for pushing a ref in Game 2, scored five points on 1-for-10 shooting. She was 1-for-8 from 3-point range with the one make snapping the Sky's second-quarter 10-0 run. Bria Hartley had five points in the first quarter that the Sky won, 20-11.

No one on the Mercury other than Griner hit double figures and no one had double-digit rebounds. Brianna Turner had a team-high seven rebounds. Alanna Smith had seven points and seven rebounds off the bench, filling late minutes with the game already done.

The Mercury shot 25.8% overall (16-for-62), the lowest for a team in WNBA history and third-worst for any playoff team via Across the Timeline, and 25% from 3-point range (5-for-20). Their 50 points only barely avoided the record for lowest-scoring game of the Finals. That was set at 47 by the New York Liberty in 1999, via Across the Timeline.

Electric atmosphere in Chicago

Wintrust Arena sold out days ago and fans were seeing $1,000 resell tickets for Game 3. It's the Sky's first WNBA Finals berth since 2014 and the first for hometown hero Parker.

The courtside chairs were a who's-who of stars, including Chance the Rapper taking in another game. Chicago Bears quarterback Justin Fields was also in the house.

Diamond DeShields' father, Delino DeShields, was also in the house watching his daughter play in the WNBA for the first time, ESPN's Holly Rowe reported. The elder DeShields is a coach with the Cincinnati Reds and the two seasons overlap.  

A sellout is expected again for Game 4.

2021 WNBA Finals schedule

Game 1: Sky 91, Mercury 77

Game 2: Mercury 91, Sky 86

Game 3: Sky 86, Mercury 50 (Sky lead series, 2-1)

Game 4: Mercury at Sky, 3 p.m. ET Sunday (ESPN)

Game 5: Sky at Mercury, 9 p.m. ET Tuesday (ESPN2) (if necessary)