Both clubs have spent at least 40 days atop their division, only to now be sitting in the middle of the pack.
The Indians, winners of 10 of their last 18, have not been below .500 since April 14.
The O's, who have lost 17 of their last 26 even after two consecutive wins over Minnesota, have been .500 or better all year.
Trying to keep the O's on the plus-side of .500 will be Miguel Gonzalez, who is set to make his third start of the season.
Gonzalez, originally promoted from Triple-A Norfolk to provide long relief, has been a pleasant surprise in his two previous starts as a member of Baltimore's decimated rotation, posting a 2.84 ERA.
In his most recent outing, on Sunday against Detroit, Gonzalez allowed three runs on six hits and five walks in 5 2/3 innings.
"I kept the team in the game. That's what I'm trying to do," Gonzalez said afterward. "I felt good. Good pitches. They got hit. That's just baseball."
Much like the two teams, Indians starter Derek Lowe has begun to backslide after an impressive opening to the season.
In his last 10 outings, the 39-year-old ground-ball pitcher is 2-6 with a 6.71 ERA, raising his ERA on the season from 2.05 to 4.43.
On Sunday against Toronto, Lowe allowed three runs on three hits and four walks in six innings, making a few mechanical adjustments to face the minimum over his final three frames after surrendering all three runs in the third inning.
"That is part of trying to compete -- I imagine hitters do the same thing," Lowe said. "Change the way you are holding a certain pitch, turn slightly different on the mound. It's fun pitching like that, I have done it my whole career."
"This isn't the first time, and it won't be the last. You just have to try to get them out and try to figure it out for the next start."
Orioles: Thome heads back to Cleveland
After spending the first 13 years of his career with the Indians, Jim Thome will return to Cleveland for the first time as a member of the Orioles on Friday. Since being acquired from the Phillies via trade on June 30, Thome is hitting .238/.347/.286 with two RBIs.
The Orioles are 18-6 in one-run games, including 11-2 on the road.
The O's are 23-1 when their starter goes seven or more innings.
Indians: Damon an Orioles killer
Johnny Damon is hitting only .222 with four homers and 17 RBIs this season, but he could be in for a turnaround against one of his favorite opponents. In 222 career games against the Orioles, Damon has hit .291. His 33 home runs and 128 RBIs against the Orioles are his most against any team. Damon has played twice against the O's in 2012, going 1-for-7 with a homer and three RBIs.
After not being shut out for 57 games to open the year, the Indians have been shut out four times in the last 35 games, three times in the last 14 games and twice in the last five games.
The Indians lead the all-time series 1,114-841. Cleveland is 24-15 in the series since 2007 and has won nine of the last 12, including four in a row and seven of the last nine at home. The Indians took three of four in Baltimore from June 28-July 1.
Thome is 2-for-13 with three walks and six strikeouts in his career against Lowe.
The Indians are 2-41 when trailing after seven innings. Orioles are 35-0 when leading after seven innings.