The 31-year-old right-hander became the first Giants pitcher to throw five innings this spring, giving up just four hits and keeping the Cincinnati Reds off the scoreboard in a 6-0 victory on Saturday.
He is a non-roster invitee, signed to a minor-league contract on Feb. 10, shortly before pitchers and catchers reported to camp. He began his career as a middle reliever, pitching with the Chicago Cubs, Florida and Kansas City before becoming a starter with St. Louis from 2007-09.
"I like starting," he said. "My best season was as a starter (13-9 in 32 starts with St. Louis in 2008). I like to get out there and get a good pace going.
"I felt I pitched well. I wanted to go five, and I am glad I got to do it."
Wellemeyer is 2-0 with a 1.20 ERA in five spring games, including two starts.
Madison Bumgarner, a 20-year-old left-hander, also is in the running for the No. 5 starter's spot. He pitched in a minor league game on Saturday against the Los Angeles Angels, giving up five runs -- four earned -- on five hits in four innings.
"He (Wellemeyer) has thrown well all spring," said Giants manager Bruce Bochy. "He attacked the zone, had great command. That's what you want to see, pounding the strike zone."
Veteran first baseman Aubrey Huff helped Wellemeyer both offensively and defensively.
He hit a two-run home run, his third of the spring, in the first inning, highlighting a three-run outburst that also included a solo shot by shortstop Edgar Renteria.
He struck out Jonny Gomes, and Juan Francisco hit a sharp grounder that appeared to be headed down the right-field line. But Huff, who spent most of his time in the American League as a DH, dived to his left to snag the ball and tagged the bag. He got up and threw to second to get Votto for the inning-ending out.
"Mark DeRosa (third baseman) came up to me and said to get a strikeout and a double play and we would get out of it, and that's what happened," Wellemeyer said.
"Huff made a great play," said Bochy. "He has had a great spring. He has been swinging well. He is aggressive, looks for his pitch. He is a smart hitter."
Huff is hitting .400 (12 for 30) with three home runs and nine RBIs.
Left-hander Travis Wood was making his first start and fourth appearance for the Reds, who made him their minor league pitcher of the year in 2009. He gave up three runs -- two earned -- in three innings, walking two and striking out four.
"I'd like to have had a better outing," Wood said. "You are going to get hit. It is going to happen. You have to keep your head on straight. I try not to think about making the team. That adds pressure to what's already there. I just go out and pitch."
Wood is competing for the fifth starter's spot with right-hander Mike Leake, the team's first-round draft pick and a former star at nearby Arizona State. Leake gave up three earned runs on seven hits in three innings against the Giants.
Two-time National League Cy Young Award winner Tim Lincecum is scheduled to pitch in a minor-league game on Sunday while the Giants travel to Tucson to play Arizona. ... After sitting out a handful of games with a strained quadriceps, OF Aaron Rowand is due to play Sunday. ... 3B Pablo Sandoval (gash in his shin) likely will return to action on Monday or Tuesday, Bochy said. ... The Reds play split-squad games Sunday against the Chicago Cubs in Goodyear, Ariz., and against the Oakland Athletics in Phoenix. Right-handers Justin Lehr and Johnny Cueto will start.