The Eastern Conference-leading Celtics had been widely expected to silence the Thunder, who were without go-to man Shai Gilgeous-Alexander (illness), who ranks among the league's leaders with 30.8 points per game.
But in Gilgeous-Alexander's absence, Australian guard Giddey scored a season-high 25 points and added five rebounds, five assists and two steals in the Thunder's 150-117 stroll on Tuesday.
Jayson Tatum drove his team to make up for that stumble, scoring his second career triple-double (29 points, 14 rebounds and 10 assists) to sink the Mavericks.
Dallas superstar Luka Doncic scored 23 points after averaging 44.5 over the previous five games.
"They showed me they can do it one time," interim Boston coach Joe Mazzulla said. "They have to do it over and over again.
"When we're at our best, we play really good basketball on both ends of the floor with high effort for a long period of time.
"And then we go through a couple of games where we lose that. What I love about tonight was we did bounce back."
"I've said it over and over again this year, it's just my teammates, the coaching staff - they put me in the right spots to succeed," Markkanen said.
"My teammates did a great job delivering me the ball and it was just finishing my shots."
Jamal Murray sparked Denver early, scoring 13 of his 18 points in the first quarter, with the Nuggets at one stage leading by as many as 43 as they cruised to a 122-91 win over the Los Angeles Clippers.
- with AP