Rookie Raisel Iglesias pitched six strong innings and Joey Votto scored the go-ahead run on a balk by Shields and later homered to lead the Cincinnati Reds to a 7-3 victory against the Padres on Wednesday.
The Padres were trying for just their second three-game sweep of the season. It was San Diego's seventh loss in nine games.
Shields (8-5), who signed a $75 million, four-year contract as a free agent in February, has just one win in his last 13 starts.
''We're not playing up to our potential,'' Shields said. ''I know it is cliche but this is a brand new ballclub. It takes time to be able to jell and have good chemistry and really have that drive. We haven't been playing good baseball all season long. We've had times where we play good baseball and times when we haven't played good baseball at all. We need to focus the rest of the season and finish strong.''
With the game tied at 3 and one out in the third, Shields allowed a one-out single by Votto, followed by a double by Todd Frazier that moved Votto to third. The next at-bat, it appeared Shields faked a pickoff throw to third, and he was called for a balk, allowing Votto to score the go-ahead run. It turned out that third baseman Yangervis Solarte missed a sign and was playing back, so Shields didn't make the throw.
After walking the bases loaded and then getting out of the inning, Shields glared at plate umpire Gerry Davis as he walked to the dugout. He then raised both arms in a gesture to someone in the dugout.
''Votto wasn't really paying attention over there and I felt like in that situation we had him and now we got two outs,'' Shields said.
''He thought Soly had the sign and obviously that didn't happen,'' Murphy said.
''You give up a bunch of hits and that happens,'' Shields said. ''Mental mistakes just can't happen. We've got to clean that up. I mean those are the things we can control on our side and we definitely have to clean that up.''
Shields allowed five runs and six hits in 6 1-3 innings, striking out seven and walking four. He left after loading the bases in the seventh. Shawn Kelley came on and allowed a sacrifice fly to Frazier.
Iglesias (3-4) retired 16 straight from the end of the first inning through the sixth to help the Reds avoid a three-game sweep. He allowed three runs and six hits while striking out eight and walking one. He had lost four of his previous five decisions.
Votto hit a two-run homer - his 20th - off Frank Garces with one out in the ninth. Votto had three hits and drove in three runs.
Shields fell behind 3-0 in the first, when four of the first five Reds batters reached base. Votto hit an RBI single and Jay Bruce hit a two-run triple off the top of the center field fence.
Matt Kemp tied it in the bottom of the inning with a three-run homer to left, his 12th. Leadoff batter Solarte reached when he was hit in the ribs by a pitch and Yonder Alonso followed with a single to left before Kemp's homer.
Reds: Cincinnati heads up the freeway for a four-game series against the Los Angeles Dodgers starting Thursday night when RHP Keyvius Sampson (1-1, 3.00 ERA) is scheduled to start against Mat Latos (4-8, 4.67).