The path was cleared for Cantu to be in the lineup on March 31 after Jose Castillo was claimed off waivers by the Giants on Saturday afternoon.
The Marlins entered Spring Training throwing the competition up for grabs. Cantu, a non-roster invitee who previously posted monster power numbers, was in the mix with Castillo and Dallas McPherson.
Castillo, however, was placed on waivers on Thursday, and he was headed to Giants camp as of late Saturday night. McPherson, meanwhile, has nice upside and power potential, but he's missed substantial time this spring because of injury. McPherson was in the starting lineup, playing third base on Sunday in Grapefruit League action against the Nationals.
Based on performance, Cantu is winning the job with his bat. His glove, while not as flashy as Castillo's, has shown promise.
"A lot can happen. It ain't over yet," Cantu said of his status with the club. "I came over here to get my bat alive again, and get that going. My main goal was to come in and compete for the third-base job. There is still competition out there."
But he concedes: "Everything is going in the right direction. I came into Spring Training working hard. I worked hard in the offseason with a personal trainer in Houston. It's paying off."
In Houston, Cantu worked out under the supervision of Dennis Fay at the Texas Sports Medicine Center. Adam Dunn of the Reds and several other big leaguers also were there.
Through intense training, Cantu raised his weight from 190 pounds to 210.
"It's all lean muscle," he said. "I've gotten a little bigger."
According to baseball insiders who have followed Cantu the past few years, they say his body looks like it did back in 2005, when he enjoyed a huge year with the Rays. In 150 games, he batted .286 with 28 home runs and 117 RBIs.
Cantu entered Sunday with a .431 batting average, which ranked second overall in Spring Training.
After it was announced on Saturday that Castillo was heading to the Giants, Marlins president of baseball operations Larry Beinfest said: "Cantu had a leg up on the competition."
That's a main reason why Florida gauged to see the level of interest Castillo would have on waivers.
For Cantu, he came into camp determined to prove he still could be a productive player. After his strong 2005 season, his 2006 season was hindered because of injuries. He went on the disabled list with a fractured bone in his left foot after he fouled a ball off it.
In 2007, he opened the season with the Rays, but finished up with the Reds. In those two stints, he combined to hit .252 with one home run and 13 RBIs in 52 games. He also played 48 games at the Minor League level.
It was a humbling season.
"It's motivating because a lot of people start doubting you, but inside, you believe in yourself," Cantu said. "You believe you are able to do everything over again, because a little injury backed you off it a little bit. In the end, to myself, I feel I put decent numbers up there, and had a pretty good season. Last year, I didn't have a chance to play [in the big leagues]. That's it. Plain and simple.
"That gets you down a little bit, and you ask, 'What did I do?' That's the baseball business. But it is humbling to a point, but you know this is Major League Baseball. This is the best baseball there is. I'm not taking that for granted. Everybody knows what I can do."
Third base has been one of the most closely watched positions in Marlins camp. The position opened after Miguel Cabrera was traded to the Tigers in December.
The Marlins aren't looking for any one player to pick up the slack now that Cabrera's immense production is out of the lineup. But Cantu had a taste of a Cabrera-like season in '05.
"Look at Miguel Cabrera's numbers. Those are really solid numbers every year, and I had a season like that," Cantu said. "I know in my heart I can do it again if I got a chance to play. That's why I'm here. I love this team. I love these guys, they hustle every day, and it's fun."
Joe Frisaro is a reporter for MLB. This story was not subject to the approval of Major League Baseball or its clubs.