Ross (3-0) allowed one run on six hits in six innings, raising his ERA to 0.79. He has given up only two runs in 22 2/3 innings.
Werth added an RBI single in the eighth, his fifth in the first two games of a weekend series and 10th of the year. Washington goes for a sweep Sunday after raising its record to 8-24 at 11-year-old Busch Stadium.
Opponents had been 2 for 13 with no runs in the first against Jaime Garcia (1-2) before the Nationals jumped on the lefty for four runs on three hits. Daniel Murphy had an RBI single before Werth hit his fourth over the left-field wall.
Garcia struck out six and walked three in 6 1/3 innings.
