Matt Holliday belted a three-run homer to key a six-run fourth-inning outburst to send the expansion club into baseball's classic for the first time.

"This is pretty incredible," Holliday told reporters after being selected the NLCS most valuable player.

The win was the 21st in 22 games for the Rockies, who have won all seven of their postseason games including four successive victories over the Diamondbacks in the National League Championship Series.

The Rockies will play either the Cleveland Indians or the Boston Red Sox in the World Series beginning October 24.

The Indians beat the Red Sox 4-2 on Monday to take a 2-1 lead in their American League Championship Series.

Holliday staked the Rockies to a 6-1 lead and they survived a three-run homer by Arizona's Chris Snyder to win their 10th straight game, including the regular season.

GREAT RIDE

"It was fun, this has been a great ride," Rockies first baseman Todd Helton told reporters when asked about Colorado's late run.

Helton said there was more to come.

"We're not done yet. We're going to keep it going."

Arizona took a 1-0 lead on Conor Jackson's third-inning single in the do-or-die game for the Diamondbacks before the Rockies took command in the fourth.

Seth Smith's two-run double put Colorado ahead 2-1, and the runs just kept coming.

Kaz Matsui made it a 3-1 game with a single to center before Holliday followed with his long homer.

But just when it looked like the Rockies were in firm control, Arizona rebounded with three runs in the eighth to make for a tension-filled ending.

Matt Herges picked up the win in relief of Rockies starter Franklin Morales, who gave up five hits and a run in four innings.

Starter Micah Owings, who gave up the big fourth-inning hits to Colorado, took the loss for Arizona.

SPARKLING PITCHING

* Jake Westbrook delivered a sparkling pitching performance and Kenny Lofton cracked a two-run homer to help Cleveland beat Boston.

Riding the momentum from an 11-inning win in Game Two in Boston, Cleveland pushed across two runs in the second and fifth innings to propel them to a 2-1 lead in the best-of-seven American League series. Game Four is on Tuesday, again at Jacobs Field.

Westbrook, who missed six weeks of the season on the disabled list with an abdominal injury, held the Red Sox scoreless through six innings before Jason Varitek connected for a two-run shot in the seventh.

Lofton put the Indians ahead when he tagged Boston starter Daisuke Matsuzaka for a two-run, second-inning blast. The 40-year-old outfielder became the seventh-oldest player in Major League history to hit a post-season home run.

"He (Lofton) is a big game player, he likes the big stage," Cleveland manager Eric Wedge told reporters.

Matsuzaka, the Red Sox' prize $52 million off-season signing, got Game Three off to an impressive start, striking out two of the first three batters he faced.

But the Japanese righthander was quickly in trouble in the second.

Ryan Garko lined a single into centre before Lofton connected on a two out offering that narrowly cleared the wall in deep right for his seventh career post-season homer.

(Writing by Gene Cherry in Salvo, North Carolina)