Major-league saves leader Jim Johnson, roughed up for five runs in a Game 1 loss, pitched a perfect ninth to close it out.
Game 3 of the best-of-five series will be played at Yankee Stadium on Wednesday.
For the second day in a row, the start at Camden Yards was delayed by rain, this time for 40 minutes.
Chen, making his first postseason start, allowed one run and six hits over the first six innings. The Taiwanese native, who pitched previously in Japan, was 1-2 with a 5.25 ERA in four outings against New York this season, including two in September in which he yielded a total of 11 runs over 111/3 innings.
Pettitte, whose 19 wins are the most in postseason history, gave up three runs and seven hits in seven innings.
Baserunning was a key early in the game. Ichiro Suzuki danced around Orioles catcher Matt Wieters to score, while Baltimore's J.J. Hardy failed to see his third-base coach and was left stranded.
Pettitte retired the first eight batters before Robert Andino hit a bloop single with two outs in the third. Nate McLouth also singled, and a four-pitch walk to Hardy loaded the bases for Davis, who lined a single to right.
Adam Jones followed by grounding a single just beyond the reach of shortstop Derek Jeter, but Hardy stopped at third after failing to spot third-base coach DeMarlo Hale waving him home. Wieters then popped out with the bases loaded.
Wieters led off the sixth with a double and scored on a single by Mark Reynolds to make it 3-1.
The Yankees used the deft footwork of Suzuki to take a 1-0 first-inning lead, and it had nothing to do with his speed on the basepaths.
Jeter led off the game with a single and Suzuki reached when Reynolds fumbled a barehanded pickup at first base for an error.
Alex Rodriguez hit a low line drive at Andino, and the second baseman caught it and doubled up Jeter. Robinson Cano followed with a drive to the base of the right-field wall for a double. The relay from Davis to Andino to Wieters beat Suzuki to the plate by plenty, but he dodged the tag coming toward home.
Suzuki then circled around the batter's box, juked around the catcher's desperate lunge and touched the plate an instant before Wieters' glove found its mark.
Suzuki also hit an infield single in the third inning, extending his hitting streak at Camden Yards to 21 games.