Albuquerque (72-65) pulled within six games of Oklahoma City (78-59), a 6-3 loser to Nashville, with seven games left to play.
"The goal is to win every night, whether you're in it or out of it," said center fielder Matt Angle, who went 2-for-4 with two runs scored. "I think when you stop trying to win it might be time to hang 'em up."
The Isotopes took advantage of three errors by the Express (71-66) as only two of their six runs were earned.
"It was a pretty good ballgame, we did some good things," manager Lorenzo Bundy said.
Albuquerque scored what proved to be the winning run in the sixth inning. Elian Herrera reached on an infield single and took second base on a throwing error by third baseman Alex Buchholz. Herrera stole third base and then scored on Tony Gwynn Jr.'s single to left field to make it 6-3.
Round Rock climbed back within a run as Aaron Cunningham blasted a two-run homer off the left field foul pole in the seventh inning.
Isotopes starter Jonathan Sanchez (6-3) stayed in two batters too long as he walked Robinson Chirinos before giving up Cunningham's shot.
Sanchez picked up the win despite allowing five runs on six hits and two walks while striking out eight in six-plus innings.
"I think sometimes (Sanchez) gets caught up in worrying about maybe not walking somebody, sometimes he gets caught up in just throwing a strike rather than making a quality pitch," Bundy said.
Relievers Kelvin de la Cruz, Peter Moylan and Javy Guerra then pitched a scoreless inning apiece, with Guerra striking out Engel Beltre to end the game and earn his 11th save.
"Nice job by the bullpen other than the two leadoff walks, the first hitters that De la Cruz and Peter faced," Bundy said. "Javy did a nice job in the ninth."
Albuquerque struck first on Chili Buss' two-out, two-run double off the center field fence in the first inning. Both of those runs were unearned as Tony Gwynn Jr. reached safely on a sacrifice bunt thanks to a throwing error by catcher Chirinos that pulled first baseman Chris McGuinness off the bag.
Leading 3-1 in the third, the Isotopes put two on and executed a double steal. Chirinos tried to throw out Angle at second base but no one bothered to cover the bag and the ball went into center field, enabling Buss to score and Angle to reach third. Justin Sellers' sacrifice fly then drove in Angle to make it 5-1.
"And I didn't realize (no one was covering second); Chili said no one was covering third, either," Angle said. "We didn't know what was going on."
McGuinness kept Round Rock alive with a two-run homer off Sanchez in the fourth inning.
"Sanchy was good but pretty much McGuinness and Cunningham cost him five runs," Bundy said. "Other than them he kept that lineup in check."
The Isotopes and Express will meet again Tuesday at 6:35 p.m.
Around the Lab
- A total of 6,800 fans consumed 16,061 hot dogs on 50-cent night, an average of 2.36 per fan.
- Buss was named to the postseason All-Pacific Coast League Team as an outfielder. He was the lone Isotope represented on the squad. Buss leads the PCL with 96 RBI. He was previously named the Isotopes' MVP and Fan Favorite on Sunday.