Bumgarner and Gray go toe-to-toe as the Bay Bridge Series continues Tuesday night in Oakland.
Bumgarner (9-6, 3.09 ERA) had won six straight decisions before going 1-3 with a 4.09 ERA in his last five starts. After yielding a season-high five earned runs over six innings in a 6-2 loss to Cincinnati on June 27, the left-hander allowed five runs - four earned - over five innings in Thursday's 7-2 defeat to St. Louis.
"It just wasn't happening," he said. "I wish I had the answers."
Getting back on the road could help Bumgarner, who's earned a career-best five straight victories away from home behind a 1.05 ERA. He hasn't been very sharp in two career starts against the A's, though, going 1-1 with a 5.68 ERA.
Gray (8-3, 3.08) had gone 2-2 with a 5.50 ERA over a six-start stretch before allowing one run and four hits over seven innings Thursday in a 4-1 win over Toronto.
"His curveball was disgusting," catcher Stephen Vogt told MLB's official website. "It was back to the Sonny Gray that I've known for the last year and a half. He's got one of the best curveballs in baseball, and he had it tonight."
Opponents are hitting .183 against Gray's curve, hardly impressive, but better than a .111 average a year ago.
"That was the idea, to really get that going again," Gray said of his curveball. "I think the last five or six starts it's been OK, but it hasn't been a big factor. ... I was able to find that again."
This marks Gray's first appearance against the Giants (49-40), who have dropped eight of 11 following Monday's 5-0 defeat. Hunter Pence saw his 11-game hitting streak snapped as San Francisco was limited to five singles, going 0 for 2 with runners in scoring position.
''We have to get back to the team we were when everybody did something to help us win,'' manager Bruce Bochy said. ''We have guys cold with the bats. We have to get that confidence back, the belief it's going to happen. We need to get our spark plugs clean and get them back ready to go.''
The MLB-leading A's (56-33) are now on the verge of a season best-tying sixth straight win. Alberto Callaspo went 2 for 4 with two RBIs Monday and Jesse Chavez threw six strong innings.
Oakland won six in a row May 7-13.
"It's exciting to be part of the Oakland Athletics right now," said third baseman Josh Donaldson, one of six A's players to make the All-Star team.
Oakland, which has taken five of six from the Giants, has outscored teams 49-25 in winning 10 of 11 on its own field.
"We're just playing good baseball, and we're excited to be back home," Chavez said.
San Francisco, meanwhile, has dropped 12 of 14 road games against the AL dating to June 2012, including four straight in Oakland.
Giants third baseman Pablo Sandoval hasn't played since exiting Saturday's game after being hit by a pitch on his left elbow, leaving his status for Tuesday in doubt. He was hitting .190 during a 12-game stretch before getting hurt.
The Giants and A's play two more in San Francisco after this game.