Washington has won three straight, 10 of 11 overall and are an Eastern Conference-best 17-5 since New Years Day.
"It definitely starts with him," Beal said of shutting down Westbrook. "Containing him as much as possible, keeping him out of the paint. Make him take a lot of tough jumpers.
"That's the game we want him to play. He wasn't comfortable with it. We made it difficult for him and their whole offense to get going."
Westbrook scored 17 points for the Thunder, but the NBA Most Valuable Player candidate was a long way from his triple-double form.
"I was doing what I wanted. I just missed," said Westbrook. The point guard played just 24 minutes.
It has been quite a week for Westbrook. Less than 48 hours after an emotional matchup against former his teammate, Golden State Warriors star Kevin Durant, he finished with four rebounds and four assists while shooting five of 19 from the field.
The road matchup against the Wizards pitted all-star point guards Westbrook and Wall against each other.
It didn't garner the same amount of attention that came from Durant's first game in Oklahoma City since spurning the Thunder for the Western Conference champion Warriors.
But the same outcome means two losses in a row for the Thunder, who fell to 31-25, shooting just 35.4 percent overall on Monday.
"He had an off night," said Washington coach Scott Brooks, who coached Westbrook in Oklahoma City for the seven seasons. "He's allowed to do that. It doesn't happen often."
- Spurs beat Pacers -
Elsewhere, Kawhi Leonard tallied a game-high 32 points as the San Antonio Spurs clinched their 20th straight winning season with a 110-106 win over the Indiana Pacers on Monday.
The Spurs have lost two in a row only twice this NBA season and are 9-1 on the second night of consecutive games.
Aldridge's three-point play with 52 seconds left gave the Spurs a 105-100 lead, and Parker made two free throws with 23 seconds remaining that bumped the lead to 107-101 at Bankers Life Fieldhouse arena.
Two Aldridge free throws at the 3:55 mark and Leonard's baseline jumper with 3:34 to play gave San Antonio a 98-97 lead. Another Leonard basket stretched the lead to 100-97 with just under three minutes left.
Paul George scored a team-high 27 points for Indiana, who dropped to 29-26 on the season.
Myles Turner finished with 22, Jeff Teague scored 15 and Rodney Stuckey added 13 for the Pacers, who lost their fourth straight game.
San Antonio outscored Indiana 21-12 to close the second quarter as the teams went into the half tied 49-49.
- Denver topples Warriors -
In Denver, Juancho Hernangomez nailed six of Denver's franchise-record 24 three-pointers as the Nuggets blasted the Golden State Warriors 132-110.
Nikola Jokic had 17 points, 21 rebounds and 12 assists for his second triple-double. He set career highs for assists and rebounds but was overshadowed by the team's outside shooting as four players hit at least four three-pointers,
Kevin Durant scored 25 points for Golden State. The Warriors were just eight of 32 from three point range, including Stephen Curry's one-of-11. Curry went four of 18 from the field and finished with just 11 points.