Harden won the duel, pitching seven solid innings Saturday night in a 2-1 victory over the Los Angeles Angels.
"It feels good to get back out there on the mound," Harden said. "I've been thinking about getting out there for a while. It's hard not to think about it and get that first win out of the way."
Vladimir Guerrero helped beat his former team with a two-run homer that ended a 16-game drought. The Rangers' lead over the Halos in the AL West moved back to nine games.
Harden (4-3) allowed a run and five hits, struck out three and walked two. The right-hander had been sidelined since June 11, when he strained his left gluteal muscle.
The Angels' run came on a seventh-inning homer by Howie Kendrick.
"He certainly was patient with us, we were patient with him, and he rewarded us tonight by going out there and giving us a well-pitched game against another pitcher who can pitch very well also," Texas manager Ron Washington said. "It was a huge boost."
Angels manager Mike Scioscia was impressed with the new and improved Harden, who is 6-3 against them.
"He's reinvented himself," Scioscia said. "He had that power fastball with a splitter. Now he's got a good changeup and he uses it effectively."
Frank Francisco pitched a perfect eighth inning and Neftali Feliz did likewise in the ninth for his 29th save in 31 attempts and 20th in a row, finishing up a game that took 2 hours, 11 minutes.
Feliz is the third rookie in major league history to convert at least 20 consecutive save opportunities, joining Oakland's Andrew Bailey (21 in 2009) and Boston's Jonathan Papelbon (20 in 2006). The longest save streak by any Texas pitcher is 21, by Francisco Cordero in 2004.
Haren (0-2) went the distance for his second complete game of the season and 11th in 210 career starts. The right-hander threw 119 pitches, allowing five hits and striking out six in his second start for the Angels after joining them on July 25 a trade that sent Joe Saunders and three minor leaguers to the Arizona Diamondbacks.
Last Monday, Haren was forced out of his Angels debut in the fifth inning because of a bruised forearm after getting struck by a line drive off the bat of Boston's Kevin Youkilis. The three-time All-Star has gone a career-worst nine consecutive starts without a victory, including three no-decisions. Haren's last win was June 12, when the Diamondbacks beat St. Louis 7-2.
"I feel better about this one than last one," Haren said. "This was a big game, and I lost. But I felt good out there and I'm happy with the way I threw. They were just a little better. I wish I had that one pitch back because I was in control the whole night."
Harden and Haren spent three seasons together in Oakland from 2005-07.
"It was definitely fun matching up against him," Harden said. "It was always fun watching him pitch back in Oakland, but our rivalry was more off the field, with video games and stuff like that. I've always tried to catch up with him when he was pitching for Arizona, and he pitched a great game tonight."
Haren was facing a Texas lineup fortified for the stretch drive by the additions of second baseman Cristian Guzman and first baseman Jorge Cantu, who were obtained prior to Saturday's non-waiver trade deadline from Washington and Florida, respectively.
Major league batting leader Josh Hamilton was scratched from the Rangers' lineup because of patella tendinitis. The three-time All-Star, hitting .362 with 23 homers and 75 RBIs, missed his fourth game of the season and first because of injury but was available to pinch-hit.
"He just didn't feel good, and he didn't want to start the ballgame and have to come out," Washington said.
David Murphy, batting third in place of Hamilton, drew a leadoff walk in the fourth and Guerrero drove Haren's next pitch to left-center for his 21st homer.
It ended a drought of 68 homerless at-bats for the nine-time All-Star since his three-run shot against Baltimore's Brian Matusz on July 9 at Arlington.
"You go through those streaks where you're not feeling it or you're swing is not as grooved as you know it could be," Guerrero said through a translator. "I just went through one of those times."
NOTES: Hamilton is batting .411 with six homers and 22 RBIs in 32 games since his last unscheduled day off on June 23. ... The Rangers finished July with a 14-13 record after going 21-6 in June -- which was their best record for any month since the franchise moved from Washington D.C. to Texas in 1972. ... Texas RF Nelson Cruz extended his career-best hitting streak to 19 games with a sixth-inning single. ... Texas is 20-10 against AL West rivals.