HOUSTON -- After a blistering start to the year, J.D. Martinez is working through his first slump.

Entering Friday's game against the Cardinals, the powerful Houston left fielder had a batting average of .105 in his last six games, with only a pair of singles in 23 plate appearances. That's dropped his average for the young season from .313 to .267.

But Martinez doesn't appear to be pressing, with several hard-hit balls of late. He's also maintained his plate discipline, walking four times in that six-game stretch.

Manager Brad Mills doesn't appear overly concerned, either. He stuck with Martinez as the No.3 hitter in Friday's lineup, which is where he's been for almost the entire season.

"J.D. is one of those guys that works on things every day," said Mills. "I don't care if he's hitting .500 for the last month and a half. He goes out and always works, always tries to get better. That's just him.

"He was in the cage early this afternoon working on some things. What they were particularly, I'll let him and our hitting coach [Mike Barnett] take care of that.

"I only get concerned if it feels like it's wearing him down."

The struggles haven't been to the detriment of the Astros, who won four of those six games.

Schafer reaches base again, ties Astros record

HOUSTON -- Two days ago, Astros center fielder Jordan Schafer was wincing in pain, grabbing his lower back after a strikeout against the Mets and leaving the game prematurely.

In the Astros' 5-4 win against the Cardinals on Friday, things appeared just fine as he tied a club record.

Schafer's eighth-inning walk extended his streak of reaching base safely (not counting errors) to 25 games to start the season. That ties him with Denis Menke (1969) for the Astros record.

"He sets the table for us," said manager Brad Mills. "He does what we need him to do."

Schafer reached twice earlier in the game on errors, scoring both times. He also stole a pair of bases.

He'll likely have a chance to set a new club mark on Saturday night against St. Louis left-hander Jaime Garcia. Schafer appeared in good spirits before and after the game, and Mills said he expected no long-term effects from Schafer's recent back issue.

"He seems fine," said Mills. "I asked him how he was doing and he said it was fine. The trainer said there was no problem."

Schafer's on-base success is helping drive a remarkable turnaround for the Astros as a team. After finishing last in the Majors in walks a year ago, Houston is second in the NL in walks and on-base percentage in 2012.

Worth noting

• Rusty Staub, a member of the 1965 Astros, was scheduled to throw out the ceremonial first pitch in Friday's series opener with the Cardinals. Staub, an outfielder and first baseman, hit .273 with 57 home runs in his six seasons in Houston.

• The Astros continued their "Flashback Fridays" theme at Minute Maid Park by wearing retro shooting star uniforms from the 1960s. In celebration of the franchise's 50th anniversary, the team wears a different decade's uniform each month for its Friday home games.

• Second baseman Jose Altuve and shortstop Jed Lowrie entered Friday with the highest combined average of any middle-infield combination in the Majors, batting .332.