Cubs 5, Cardinals 1: Logan Watkins and Chris Valaika belted back-to-back homers to lead off the fourth and Felix Doubront tossed seven solid innings as visiting Chicago won the opening game of a doubleheader.

Starlin Castro matched his career high with his 14th homer for the Cubs, who have won the first two contests of this four-game series and six of their last eight overall. Chris Coghlan had a two-run single and Watkins and Valaika each scored twice in the victory.

Jhonny Peralta scored on a double play for the Cardinals, who have dropped four in a row to fall two games behind first-place Milwaukee in the National League Central and saw their lead dip to a half-game over Pittsburgh in the race for the second wild card. Justin Masterson (6-9) continued to struggle with his new team, permitting five runs on six hits in 4 1/3 innings to fall to 2-3 since being acquired from Cleveland.

With Chicago nursing a 2-1 lead in the fourth inning, Watkins deposited a first-pitch fastball from Masterson over the wall in right for his first career homer. Valaika followed and sent an 0-1 slider into the seats in left-center field for his second blast of the season before Castro matched his 2012 total with a solo shot in the fifth.

Doubront (3-4) excelled in his debut with the Cubs by allowing one run on seven hits while striking out four. Neil Ramirez walked and struck out a batter in a scoreless eighth and Justin Grimm retired all three batters he faced in the ninth.

GAME NOTEBOOK: Chicago drew first blood in the second as Coghlan's single plated Watkins before St. Louis RF Randal Grichuk initially threw out Valaika at the plate to end the inning. The Cubs challenged the play and the officials overturned their initial call. ... Chicago 1B Anthony Rizzo missed his fourth straight game with lingering tightness in his lower back. He is not expected to play this weekend. ... Cubs RHP Edwin Jackson threw a light bullpen session on Saturday as he attempts to work his way back from a right lat strain.