Cincinnati (49-36) is 1-8 all-time against Seattle (37-48), with the most notable connection between these franchises being the clubs Ken Griffey Jr. played the bulk of his career with.
A less notable connection from a Seattle standpoint is Harang (3-7, 5.08 ERA), who is in his first season with the Mariners but went 75-80 with a 4.28 ERA from 2003-10 for Cincinnati.
Harang won his first start against the Reds since leaving the club by allowing one run over seven innings last July 4 in a 4-1 victory for the Dodgers, but he has yet to return to Cincinnati.
"I like Harang," manager Dusty Baker told the Reds' official website. "Harang was a big part of us rebuilding this. I'm sure people will like seeing him. We haven't seen him (in Cincinnati) in a while."
The veteran right-hander is 0-4 with a 6.75 ERA in five road starts.
Harang will be up against another righty born in San Diego in Leake (7-3, 2.52), who was scheduled to pitch Thursday against San Francisco before that contest was postponed due to rain.
Leake is 2-0 with a 1.50 ERA in his last four outings, yielding four runs - one earned - over seven innings in Saturday's 6-4, 11-inning victory at Texas.
He has never faced Seattle, and is 2-0 with a 2.36 ERA in his last seven interleague starts.
The Reds have won three straight by yielding three runs over 26 innings in that span, including Homer Bailey's no-hitter Tuesday against the Giants.
Their offense continues to struggle, averaging 3.2 runs while batting .236 over the last 10 games. Their top four RBI leaders Brandon Phillips, Jay Bruce, Todd Frazier and Joey Votto - have combined for 16 in that stretch.
Seattle missed a chance at a three-game sweep in Texas with Thursday's 5-4 defeat. The Mariners blew a two-run lead in the seventh inning and fell despite a 14-8 advantage in hits after going 1 for 12 with runners in scoring position.
"We've got to do better with guys on base," said Raul Ibanez, who homered among a season-high four hits and three RBIs.
Ibanez is batting .381 with seven homers and 13 RBIs during a 10-game streak, although he is 2 for 9 against Leake.
Justin Smoak went 6 for 13 in the Rangers series.
Outfielder Michael Saunders hasn't started since a June 26 contest against Pittsburgh due to a finger injury, but he is expected to be in the lineup Friday.
"We'll get him back in there (Friday)," manager Eric Wedge told the Mariners' official website. "That's the plan right now."