SAN FRANCISCO — When the Giants visited Wrigley Field two months ago, Jerome Williams was disappointed he didn't pitch for the Cubs in the series against his former team. So he plotted out the rotation, figured out he'd pitch Sept. 11 at SBC Park, and circled it on his calendar.

Williams ended up pitching two days earlier, and it was his bad fortune to draw Matt Cain.

The Giants' 20-year-old rookie threw a two-hit complete game in his third major league start, and San Francisco spoiled Williams' homecoming in a 2-1 victory last night.

Williams, a Waipahu High graduate, was making his first appearance against the Giants since they traded him in May along with right-hander David Aardsma for reliever LaTroy Hawkins.

Williams developed a reputation as being a little hardheaded in the Giants organization. He also had elbow surgery last year that slowed his development.

But the overriding sentiment among fans was that the Giants made a mistake when they traded him, and the right-hander looked like an ace in an emotional return.

Despite being fired up from the first pitch, Williams kept his wits about him and held the Giants to two runs (one earned) over seven innings.

"I had to prove something, not to everybody else but to myself,'' Williams said. "Just that I could go out and pitch and not let anything get to me.''

Cubs manager Dusty Baker said he believes Williams has improved this season, and perhaps his chats with future Hall of Fame pitcher Greg Maddux have influenced him.

"I see them talking and I don't know what it's about, but Jerome doesn't play golf and I don't think Maddux fishes or collects puka shells,'' Baker said with a smile. "So they must be talking about something.''

The Giants scored an unearned run in the third inning after shortstop Neifi Perez made an error. Derrek Lee's home run off Cain tied the score in the fourth, then Williams (5-8) made one fatal mistake in the sixth. He left a changeup up and away to rookie Dan Ortmeier, who hit a single that snapped the tie in the sixth inning.

The rest of the night belonged to Cain (2-1), who retired 15 straight before Jerry Hairston singled to start the ninth. Cain retired the next three hitters — Perez, Lee and Jeromy Burnitz — on three pitches.

Cain, who was briefly one of Williams' teammates this year at Triple-A Fresno, also got his first big league hit in the game.

"I don't like that he got a hit off me," Williams said with a laugh. "I told him when I was at Fresno, 'Just keep pitching like you're pitching and you'll be successful for a long time.' "

Cain turned in one of the most impressive starts all season in San Francisco.

"That's definitely a different feeling to go straight to the catcher, then walk back to the mound and shake everybody's hand," said Cain, whose only previous complete game as a pro was a seven-inning outing in the minors. "I'm giving effort. I just try to stay really calm. I find that works for me. If I stay calm and relaxed, my body does what it's supposed to do."

Cain, who became the youngest debut starter for the Giants in 21 years when he lost to Colorado on Aug. 29, struck out eight.

"I have to say I haven't seen very many guys like that," said Giants manager Felipe Alou, who's 49 years older than Cain. "At 20, that delivery, that breaking ball, that power and that changeup, you have to go back to the days of Tom Seaver."

"He basically did it with his fastball," the Cubs' Lee said. "He was overpowering us."