CINCINNATI -- Matt Cain's anticipation rose as he watched Fred Lewis work the count full in Wednesday's fourth inning.

"I felt like something big was going to happen," said Cain, who had watched the Giants load the bases on Rich Aurilia's two-out infield hit, a dribbler toward third base. Mindful that a similar hit by Ryan Freel preceded Brandon Phillips' grand slam that made the difference in Cincinnati's victory Tuesday, Cain figured that Lewis could duplicate the feat.

That's exactly what Lewis did, hammering Matt Belisle's fastball into the right-field seats. Lewis' grand slam capped a five-run fourth inning that propelled the Giants to a 9-5 victory over the Reds and helped end Cain's eight-game winless streak.

For a game between last-place clubs, this one was rich in satisfying achievements and compelling moments -- at least from the Giants' perspective.

Start with Lewis, who became the first rookie since the Giants moved to San Francisco in 1958 to hit two grand slams in a season.

"I'm speechless, man," Lewis said of his deed.

Lewis is striving to make a statement with his performance, although he faces the challenge of wresting playing time from the veteran outfield triumvirate of Barry Bonds, Dave Roberts and Randy Winn. Lewis, 26, could receive increased chances to prove himself if the Giants continue to struggle -- in fact, manager Bruce Bochy planned to insert Lewis somewhere in Thursday's lineup -- but he still must gain polish and consistency before he can end the organization's drought of producing capable position players.

"It's a workable situation," Bochy said, indicating that Lewis can garner an adequate number of starts when a regular outfielder needs a rest. "He certainly is forcing this issue to find ways to get him in there."

Indeed, Lewis has displayed a flair for the spectacular. Of his three homers, his only non-grand slam was his first, which he collected while hitting for the cycle on May 13 at Colorado. That happened to be Mother's Day, which Bonds shrewdly referred to when Lewis returned to the dugout following his Independence Day homer.

"He said, 'You only swing on holidays,'" related Lewis, who replaced Bonds in left field for the afternoon.

That May 13 game happened to be the date of Cain's last victory. He was 0-6 with a 3.57 ERA since then, victimized mainly by an offense that scored more than two runs for him in just five of his first 16 starts, 14 of which resulted in Giants defeats.

"I know he's been frustrated lately," Aurilia said. "He's been pitching his butt off and not getting anything for it."

Cain appeared bound for another loss after the Reds jumped ahead, 3-0, through three innings.

"We thought, 'Oh, no, another tough day for Matt,'" Bochy said. This time, however, the offense responded. So did Cain, who exhausted his pitch limit and himself after slipping a called third strike past Phillips to end an 11-pitch showdown with two Reds on base, a run in and the Giants leading 7-4 in the sixth inning.

With two outs, Steve Kline entered the game to face Ken Griffey Jr. and rescued the Giants by coaxing a fly to right field from the Reds' All-Star on a full-count changeup.

As rewarding as this might have been personally for Kline, he said, "I get more satisfaction to see the kid get over the hump."

That would be Cain (3-9), who was spared the indignity of entering the All-Star break with a double-digit loss total.

"It's been a long time since I got to enjoy [a win]," he said. "It's definitely a relief to get it out of the way."

San Francisco insured Cain's triumph by bashing a season-high four homers. Ray Durham and Aurilia went deep with two outs in the sixth off Ricky Stone, marking only the second time this season that the Giants have gone deep back-to-back. After rain halted play for 51 minutes in the middle of the eighth inning, Mark Sweeney led off the ninth against Marcus McBeth with his 15th career pinch-hit homer, tying Dave Hansen for seventh on the all-time list. It was Sweeney's fourth hit in his last eight pinch-hit at-bats.

"Hopefully I can get things going," he said, "and help the team win some games."