The Wizards are 11-4 against teams with winning records since Jan. 7.
"We just step up to the challenge," said Wizards reserve Chris Singleton. "We executed the game plan the coaches came up with and down the stretch, we're not our old selves. You see us. We grew up."
Patrick Beverley scored a career-high 15 points with three 3-pointers and Delfino had four 3-pointers and scored 12 points for the Rockets.
McHale limited Jeremy Lin, who scored five points, to 26 minutes due to illness.
The Rockets hit from long range in the first quarter, sinking 8 of 14 3s for a 32-22 lead. Parsons made his three attempts and Harden added one during an 11-4 run to close the quarter.
Three more 3-pointers came during a 14-0 spurt as Harden's trey put Houston ahead 48-31.
The Rockets led 57-46 at halftime, but the Wizards fought back with a 19-6 run highlighted by seven points from Ariza and Wall's jump shot for a 73-72 lead with 3:14 remaining in the quarter. The Rockets regained the lead and entered the fourth quarter up 83-77. Houston missed seven of 10 3's in the fourth.
"We shoot a lot of 3s as a team, but I think we got a little 3 happy," McHale said. "We didn't get enough penetration and enough ball movement."
Wall and Okafor each posted double-doubles in Friday's 119-113 win over Denver.
Houston won the first meeting at home 99-93 on Dec. 12.
NOTES: Dallas attempted 49 3-pointers on March 5, 1996. ... During the same successful stretch against teams with winning records, the Wizards are only 2-5 against sub-.500 teams. ... Veteran Jason Collins, acquired on Thursday from the Celtics, joined Washington on Saturday. Asked pregame how he has stuck around the league, the 34-year-old journeyman known for defense said, "Twelve years of being able to foul people. The stats aren't pretty. I do what I do out there." ... Thomas Robinson, Tyler Honeycutt and guard Francisco Garcia, acquired from the Kings on Tuesday, have not been cleared to play for the Rockets.