After every point, Martha’s Vineyard Regional High School seniors and tennis doubles partners Kat Roberts and Lizzie Williamson touch rackets and give each other a high-five. It’s a way of showing they have each other’s backs, no matter where the ball falls next. It’s been that way since they first started playing together as young kids at Vineyard Youth Tennis.

They are inseparable, on and off the court.

“We’re like always together,” said Williamson as she and Roberts, both 18-years-old, sat next to each other on an Airport Fitness couch Monday afternoon after practice. Williamson said the two practice together every day, are in almost all the same classes and are even planning a joint graduation party.

A few years ago when Williamson needed to pick out a new racket, she said she tried out almost a dozen before finding the right one. It was the one Roberts was using.

Duo started playing together as young kids at Vineyard Youth Tennis. — Ray Ewing

The team typically practices outside, but bad weather moved the team indoors on Monday. And skipping a practice isn’t an option as the team prepares for the postseason. The upcoming matches will be the last in storied high school careers for Roberts and Williamson, which include three consecutive team state championships. The pair is undefeated this year, and Williamson’s overall high school record thus far is 85-0. Roberts is currently 75-1.

Winning a fourth state championship will be a challenge, though, as the team moved up from Division III to Division II for postseason play this year. The team is ranked No. 2 behind Nauset, a team that they have beaten before, the pair quickly points out.

Since making the varsity team as freshmen, both players have hardly lost any sets let alone matches in the last four years. Roberts quickly impressed her coaches Nina Bramhall and Liz Roberts (her mom and sometimes doubles partner) and rose to the first singles spot during her sophomore year.

Williamson played singles at first but found her true talent in doubles, partnering with Victoria Scott for the last two years.

“This year we decided to switch it up, and it’s been so much fun,” Williamson said.

“I think our games work really well together,” added Roberts.

The pair both picked up rackets at age three and started playing doubles together when they were 10-years-old and training at Vineyard Youth Tennis. There they made up code words for communicating on the court. “Marathon” meant run to the net. “Rainbow” meant lob.

The two have since moved on from a secret language to superstitions, a sports staple. Roberts bounces the ball nine times before each serve, six times before the line and three times at the line. She said the number doesn’t signify anything, it’s just routine at this point. She also doesn’t drink water the first time she switches sides of the court.

Williamson has her own ball bouncing pattern that is too complicated to explain here. After, she keeps the ball in her right pocket, a habit she picked up from Ms. Bramhall that is rare for right-handed players.

Though they have their individual keys to success, there is also one they’ve done together since they first started playing on the same team, and feel is the most important.

“We jump up and down and do a high-five before going off to other courts,” said Williamson. “We’ve never missed it.”

The end of the year is a busy time for a senior. It’s a time of graduation ceremonies and BBQ’s, along with preparing to move off-Island for college. Roberts is graduating eighth in her class and heading to Hamilton College in New York. She said she’s not sure what she’ll study yet, maybe something in the medical field, but she will continue to play tennis for Hamilton.

Williamson is the class essayist, graduating third in the class, and will attend the University of Michigan to study English and play tennis for a club team. She said she’s also excited to pursue her other passions, dance and musical theatre, and maybe a few squirrels as part of the squirrel chasing club. (That is a real thing at the University of Michigan).

Both praise their coaches and the team they will be leaving behind. They treasure the moments in between changeovers when Ms. Bramhall and Ms. Roberts offer words of encouragement and sometimes compliment their court-style, such as the colorful visors Williamson always tries to match with her hair scrunchie.

High-school underclassmen typically send the seniors off with a class prank, and the tennis team is no exception. Still, Roberts and Williamson noticed that after their last home game no balloons or cake were waiting for them. Williamson was worried the team forgot to celebrate, but Roberts was sure they were just joking around. After two hours of holding back, the team started pelting both seniors with water balloons and dumped a bucket of water on their heads.

“We started the tradition,” said Williamson, looking back to her freshman year. “We thought it wouldn’t happen to us, and then we were both drenched by the end of the day.”

Their high-school careers may be ending, but the pair say that tennis will always be a part of their lives and helped make them into the confident young women they are today. Roberts said the sport has provided the perfect distraction on tough days and helped her build a community of friends. For Williamson, tennis has prepared her for this next step in life knowing she can rely on herself to succeed, though she knows that she can always lean on Roberts for help, including a virtual high-five whenever she needs it.

“We sometimes have almost too much fun on the court,” Williamson said. “You’ll always see us smiling and laughing.”