The rule of thumb this season has been: As the Morris brothers excel, so do the Phoenix Suns.

Friday’s loss at home to the Dallas Mavericks was a rare exception.

Markieff Morris put up 23 points and his brother, Marcus, finished with 13, but a missed P.J. Tucker 3-pointer with six seconds to go left the Suns with a 110-107 loss. Besides the Morris brothers, point guard Goran Dragic was the only other Sun to flourish in Friday’s contest, finishing with seven assists and a game-high 28 points.

The Suns entered the game 7-1 when the Morris twins combined for 30 points or more, having not lost since Nov. 6 in San Antonio when the duo reaches that plateau.

Phoenix has particularly excelled this season when Markieff Morris, the team’s 2011 lottery pick, scores at least 20. But with the outcome against Dallas, the Suns lost for just the second time in eight games when he tops the 20-point mark.

Markieff’s 23 points and 12 rebounds followed up his 24-point, 7-rebound performance in Wednesday’s home win over the Los Angeles Lakers. The third-year forward recently struggled in all but one contest of a seven-day, five-game road trip, in which the Suns went 1-4.

The mid-range jumper has been a valuable weapon for the Kansas product this season, but it became shaky during the Suns’ troublesome trip. In response, Markieff began attacking the basket more in the team’s last two games, and his scoring output improved as a result.

Markieff shot 8-for-15 in Friday’s contest and is now shooting 47 percent from the field on the season, a marked improvement from his first two years, during which his shooting hovered around 40 percent.

Even though his 3-point percentage (31 percent) is down this year, he is taking fewer long-range shots, averaging just 1.1 attempts per game (compared to 2.0 in his rookie season and 1.6 last year). Friday, he made two out of five 3-pointers against Dallas, including a four-point play with 2:30 left in the game to bring the Suns’ deficit into single digits.

Despite finishing with 107 points, the Suns had little help on the offensive end outside of Dragic and the two Morrises. Starters Gerald Green and P.J. Tucker were the only other Suns to reach double figures — 12 points and 11 points, respectively – but Green missed nine of his 13 shots in reaching that mark.

As for Marcus, he shot just 4-for-11 from the field against the Mavericks, but he grabbed seven boards in 21 minutes and made three out of his six 3-point shots, including one that brought the score to 86-84 with 9:42 left in the game — which was as close as the Suns would get before the buzzer sounded.

Friday’s loss showed that even though the Morris twins were playing in full gear, the team needs other players to contribute in order to secure a win. It also emphasized how much Phoenix is missing fourth-year guard Eric Bledsoe as an additional offensive weapon.