PITTSBURGH -- After playing 663 career Major League games without a walk-off hit, Jason Bay isn't quite ready for a new nickname yet -- even after delivering game-winning extra-inning hits two days in a row.

"Two in a career doesn't make me Mr. Walk-off," Bay said after his single in the 11th drove in Chris Gomez for a 6-5 Pirates win over the Chicago Cubs on Sunday. "But doing it back-to-back is pretty neat."

Bay's drive down the left-field line off of ex-Pirate Jon Lieber (2-3) with two men on scored Gomez from second to give the Bucs a series win against a team it had lost nine of its first 10 meetings against this season.

"He's got some very clutch hits for us all year with his home runs, tying the game for us, putting us ahead, getting us back in it, and then these last two days winning games for us," Pirates manager John Russell said. "He's that type of hitter. He can carry your club, he can get your club started."

Saturday night, Bay singled in Freddy Sanchez in the 14th inning. In both contests the Pirates tied the game in the bottom of the ninth, benefiting from a Cubs' fielding miscue each time. Saturday it was an error by Chicago shortstop Ryan Theriot that allowed Sanchez to reach. Sunday, it was Cubs left fielder Alfonso Soriano losing a Nate McLouth routine fly in the sun with Sanchez at first and two outs.

Soriano, standing just a few feet in front of the edge of the warning track near the left-field corner, shielded the sun with his glove hand but lost sight of the ball, which struck his glove but rolled out, allowing Sanchez to score.

"Off the bat, I saw it very clear," Soriano said, "but at the very last moment, when it got out of the sun, it was very tough to catch the ball because I never saw it."

A picture-perfect weather day for the 29,415 on hand had bright sun throughout. The ninth inning occurred right about the same late-afternoon time the Pirates take batting practice for a routine night game, so the Bucs were aware Soriano might have trouble with the high fly ball.

"It didn't cross my mind until I looked out there and saw him battling [the sun]," McLouth said. "We are out there every day at that time, and the sun -- especially in left field -- gets terrible."

Almost forgotten in all the late-inning drama was that Luis Rivas hit his first two home runs of the season and starter Paul Maholm got through eight innings, saving the Pirates from having to burden their weary bullpen. Xavier Nady also homered for Pittsburgh.

Rivas, who hadn't homered as a Pirate and had only two home runs combined since the end of the 2004 season, smashed two solo shots to left within the game's first three innings. Nady's blast, also to left and with none on, came in the second, and was his 11th.

"It was my first time [hitting two home runs], so it feels pretty good," Rivas said. "Now that we've got these two games here, it will make the team keep going and have more confidence in each other."

Maholm faced the minimum in five of his first seven innings, retiring the side in order four times. But he was hurt by a four-run third and ex-Pirate Aramis Ramirez's RBI single in the eighth that followed a Derrek Lee double for the go-ahead run.

He threw a season-high 121 pitches, having thrown at least 99 in each of his past six starts. In all, he allowed five earned runs on eight hits and two walks in eight innings with seven strikeouts.

"I felt good," Maholm said. "[Saturday] night was a long game, so I kind of knew I needed to go deep in a game. I told [Russell] I wanted to go a little longer than normal. Luckily I kept my pitch count down early and was able to get into the eighth and let all the guys in the bullpen rest."

For the second night in a row, the Pirates got good work from the bullpen. Franquelis Osoria worked scoreless ninth and 10th innings, and Damaso Marte (4-0) allowed only a seeing-eye single in the 11th.

"It was our two freshest arms," Russell said. "Osoria did a great job and Marte has pretty much been lights-out these last two outings. We feel real comfortable with him on the mound.

"Maholm going that deep really saves us."

Given an early two-run lead and cruising through the first two innings on only 23 pitches, Maholm appeared poised to improve on what has been an impressive record at PNC Park, but the Cubs took the lead with a four-run third.

After Rivas had made it 4-3 in the bottom of the inning, the Bucs tied it in the sixth on an RBI single by Jose Bautista, who went 2-for-4, snapping an 0-for-11 slump. Adam LaRoche had two doubles to give him seven extra-base hits in his past eight contests. McLouth had three hits and Bay two, meaning he has reached safely in 10 straight games.

"[The Cubs] have kind of had our number all year, so to [come back] against them is special," Russell said. "The thing about [our] guys is they never quit. That's the one thing about them. They battle to the last pitch, have done it all year long. They keep scratching and scraping until we win or lose. We've had a chance to build a lot of character since the season has started."