Nowitzki hit his first eight shots and scored all of Dallas' points in an 8-2 run that gave the Mavericks a 59-56 lead early in the third.
Nowitzki finally missed five minutes into the third with an errant 3-pointer.
Westbrook finished the third with a jumper that gave the Thunder a 73-72 lead.
Derek Fisher who played nine games with the Mavericks earlier in the season, played for the first time in Dallas since the team granted his release. Fisher said he was having a difficult time being away from his family and Dallas waived him on Dec. 22.
Fisher ended up signing with Oklahoma City last month and Mavericks owner Mark Cuban said before the game he was disappointed with how the situation with Fisher played out.
Dallas fans booed Fisher when he entered the game with 1:45 left. They then cheered loudly 10 seconds later when the 17-year veteran was called for his first foul.