MIAMI -- The game wasn't over after 16 minutes.

Forgive Miami and Houston if they felt otherwise.

Duncan Robinson scored 23 points, Meyers Leonard added 21 and the Heat -- fueled by one of the biggest first-quarter margins in NBA history -- ran out to a 59-18 lead on the way to an easy 129-100 win over the Houston Rockets on Sunday night.

"Surprising? Not surprising to me," Leonard said. "Don't get it twisted. The Houston Rockets are incredibly talented. But we came in with a game plan and a mindset that we were going to take away what they're good at, be disciplined and execute. That's what we did."

Jimmy Butler scored 18 points and had nine assists for the Heat, and James Johnson scored 17 in his season debut. Miami is 5-1, matching the best six-game start in franchise history.

James Harden scored 29 for Houston, while Russell Westbrook was held to only 10. Westbrook also posted a career-worst -46 plus/minus. His previous worst was -36 at Washington on Feb. 13, 2017, according to ESPN's Stats & Information.

The Heat had 38 assists to Houston's 19, and the Rockets were just 14 of 48 from 3-point range.

"We're not in trouble," Rockets coach Mike D'Antoni said after his team fell to 3-3. "But if we think we're not in trouble, then we're in trouble -- if that makes sense. You've got to have that appropriate fear and understand that it's a new year and you don't start with any games and we've got to work our way back."

It was a record-setting opening quarter for the Heat -- resulting in the third-biggest lead after 12 minutes in the NBA's shot-clock era, starting with the 1954-55 season.

Miami led 46-14 after that first quarter. The only bigger leads after one quarter since the shot clock was put into play came when the Los Angeles Lakers led the Sacramento Kings 40-4 after one quarter on Feb. 4, 1987, and the Baltimore Bullets leading the Kansas City-Omaha Kings 45-12 after one quarter on Dec. 9, 1972.

"They came out and blitzed us," Harden said. "They brought energy and effort that we couldn't match. And it showed."

It was the biggest margin by which the Heat had outscored an opponent in any quarter -- and the biggest margin by which the Rockets had ever been outscored in a period. Miami outscored New York by 28 in the first quarter on Jan. 22, 2007; the then-San Diego Rockets were outscored by 28 against Atlanta in the third quarter on Feb. 11, 1970.

The lead was 59-18 four minutes into the second quarter. Houston got within 25 points by halftime, but no closer from there.

"The energy, I don't have many complaints," Heat coach Erik Spoelstra said. "Guys really brought it tonight."

Goran Dragic scored 15 points and Tyler Herro had 12 for Miami. Chris Clemons had 16 and Ben McLemore scored 14 for Houston.

TIP-INS

Rockets: Westbrook started 1 for 9 from the field, 0 for 5 from 3-point range -- with his first make from beyond the arc coming on a 45-footer that beat the third-quarter buzzer. That got Houston within 99-71. ... The Rockets have faced deficits of at least 13 points in four consecutive games -- the other three falling well shy of the 41-point hole Houston quickly dug Sunday. ... Houston has now lost 13 of its last 17 games in Miami, and trails the all-time series with the Heat 33-32.

Heat: Miami had a 48-point quarter at New York in 1989 and a 47-point quarter at Boston in 1990. The 46-point quarter was a Heat home record. ... Chris Bosh and Ray Allen -- who teamed up for one of the biggest plays in NBA Finals history, the rebound and 3-pointer late in Game 6 of the 2013 series against San Antonio -- were in attendance. So was Dorell Wright, part of the 2006 Heat NBA championship team. ... Justise Winslow (lower back stiffness) missed his second consecutive game. Derrick Jones Jr. (left groin) and KZ Okpala (left Achilles) were inactive.

HARDEN'S SHOOTING

Harden was 6 for 14 from the field and 3 for 9 from 3-point range -- not great numbers for an elite scorer like him, but his percentages for the season actually rose. He's up to 37% from the field now through six games, 22% from 3-point range. All-time in Miami, Harden is shooting 39% from the field, 33% from beyond the arc.

CLEMONS DEBUTS

Clemons, the third-leading scorer in NCAA Division I men's history and someone who averaged 30.1 per game as a senior at Campbell last season, made his NBA debut. Clemons scored 3,225 points in 130 college games, and was 5 for 18 from the floor Sunday.

UP NEXT

Rockets: At Memphis on Monday, ending a four-game road trip.

Heat: At Denver on Tuesday, opening a three-game West swing.