Karstens capped a superb month with another strong start, Brandon Wood hit a two-run homer and the Pirates won their first interleague road series since 2003, beating the Toronto Blue Jays 6-2 on Thursday night.
The Pirates had lost 17 straight interleague series away from home since taking two of three at Tampa Bay in June 2003.
Karstens (6-4) allowed two runs and five hits in seven innings, improving to 3-0 with a 1.52 ERA in six June starts. He walked one and struck out two.
"It's been kind of surreal," Karstens said. "I've just been going out there and having fun."
Manager Clint Hurdle called Karstens' solid first half "a big unexpected pleasure."
"We thought he could come in and get us innings and win a game but the volume and depth of games he's pitching and the efficiency, walks to strikeouts, hits, his WHIP, all of it, the ERA, he's knocked the bottom out of some things," Hurdle said.
Solo home runs by J.P. Arencibia and Yunel Escobar were the only blemishes for Karstens, who has allowed five home runs in his past two starts, all solo. The right-hander has given up 16 homers this season; all but one have been solo shots.
"It's not like it's happening later in the game or earlier in the game," Karstens said. "I've just got to make some better pitches down in the zone. Usually all the home runs, except for a couple, have been up. It's one of those things I've got to work on."
As a staff, Pittsburgh pitchers have surrendered 10 home runs in the past five games. Nine of the 10 have been solo homers.
With NL Central leaders Milwaukee losing 5-0 and getting swept out of Yankee Stadium on Thursday, the Pirates are two games behind the first-place Brewers.
"Two games out going into July," outfielder Matt Diaz said. "We're really excited about that.
"We know no one else thinks we're in a race at all," he added, "but we think we are and we're hoping to be here to stay."
In a tight Central, where four teams are separated by two games, Karstens acknowledged he's already started scoreboard watching.
"Always," he said. "I was rooting for the Yankees today, I know that. It's fun, it makes the season a lot more interesting than in the past where we just kind of played to finish the year. Now we're playing because there's something to play for. The fans in Pittsburgh deserve it."
Blue Jays left-hander Brett Cecil (1-3) took the loss in his first major league start since April 20, allowing a season-high six runs and eight hits in 6 1/3 innings, the longest of his five starts this season. He walked three, one intentional, and struck out six.
"I felt like I pitched a lot better than the line showed," Cecil said.
Cecil was recalled from Triple-A Las Vegas on Monday after going 8-2 with a 5.26 ERA in 12 starts while building up his arm strength.
Cecil didn't allow another hit until the fifth, but the Pirates turned it into a run. Josh Harrison led off with a double to center, advanced to third on Michael McKenry's sacrifice and scored on Presley's sacrifice fly.
Arencibia's 12th homer and Escobar's ninth, tied it at 2-2 in the bottom half, but the Pirates reclaimed the lead with a three-run sixth.
Lyle Overbay opened the inning with a fly ball to shallow left that fell between third baseman Jose Bautista and outfielder Corey Patterson. Overbay went to second on the play, then advanced to third on Patterson's throwing error.
"It was just a mistake behind third and the floodgates opened," Cecil said.
Ronny Cedeno drove in Overbay with a double to right, then came around on Wood's homer to left, his third.
"Our pitching once again was unbelievable," Wood said. "Our defense has been great and we're starting to hit a little bit. If we put all three together, we'll be a little bit dangerous."
Pittsburgh made it 6-2 in the seventh. McCutchen chased Cecil with a double and Octavio Dotel gave up an RBI single to Diaz.
The Pirates finished interleague play with an 8-7 record. ... Blue Jays RHP Casey Janssen (forearm) will make a rehab appearance at Class-A Dunedin this weekend. RHP Dustin McGowan (shoulder) is expected to begin his rehab assignment at Dunedin this weekend. ... Toronto has homered in eight straight games, a season high, hitting 13 in that span. ... The Blue Jays have lost eight of 11 at home.