MILWAUKEE — The Heat were looking to sweep the Bucks last Sunday, but with about five minutes remaining in the third quarter, Miami’s lead was down to 4 points. LeBron James, irritated by his team’s lack of intensity on defense, punched the chair next to him during a timeout, inserted his mouth guard and methodically halted Milwaukee’s momentum.

James guarded six Bucks players, including the point guard, the rest of the way. Those six players — Monta Ellis, Luc Mbah a Moute, Brandon Jennings, Larry Sanders, Mike Dunleavy and J. J. Redick — had combined for 36 points in the first 31 minutes, but not one of them scored a basket while being covered by James.

As Miami romped through its first-round playoff series, winning all four games by double digits, the Bucks saw plenty of James’s offensive artistry as he averaged 22.7 points and 6.7 assists a game. But he also showcased his smothering defense, a side of his game that receives less acclaim but will be just as crucial if the Heat are to repeat as N.B.A. champions.

Their next opponent, the Chicago Bulls, will have to spend just as much time preparing for James’s defense as for his offense.