Facing what figures to be the Rockies' Opening Day lineup, Gonzalez allowed just two hits and one unearned run over six innings to raise his record to 4-0 and lower his ERA to 3.32.
"Each time it actually seems like he gets better," Melvin said of Gonzalez. "I don't know that he can pitch better than that. [He] continues to pitch with more pressure on him each time knowing that each time he goes out there the bar is going to be raised and he's up for the task every time."
So does he have one of the final two rotation spots sewn up?
"There's still a lot of competition left, but he's pitching very, very well," Melvin said.
With Randy Johnson set to start the year on the disabled list, there are two open rotation spots. Gonzalez has been the front-runner since camp began because of his performance last year, as well as the fact that he's out of Minor League options and would have to be placed on waivers if the team tries to send him down.
"If they name me the starter I'm going to be a starter then," Gonzalez said. "If they name me in the bullpen I do my job. We'll see what happens. I think I've won a job right now. That [Sunday's performance] helped me a lot. We'll see what happens in the next start."
Gonzalez's final spring start is set for Friday against the Padres in Peoria.
Speaking of rotation spots: It's been speculated that Enrique Gonzalez will nab the fifth spot in the rotation, but Melvin cautioned against making too many assumptions at this point.
Dana Eveland, Dustin Nippert and Micah Owings were among those mentioned at the outset of camp as candidates and have yet to be sent to Minor League camp.
But the right-hander is making things very difficult on the front office by continuing to pitch well throughout camp. Saturday against the Padres in Yuma, he allowed just two hits in four scoreless innings.
"He just continues to impress, and what I see too is a guy that's able to -- even if he doesn't start out well -- to come back and get big outs and then pitch even better after that. Those are all things that at the big-league level ... are attributes that you look for. Every time he goes out there he impresses more and more. He's still in the mix here."
What about the fact that he's not on the roster?
"Red-tape-wise, obviously there are a lot of things that play into it in the end, and to an extent [he] doesn't have that going for him, but if you pitch well, you pitch well," Melvin said. "He's one of those guys that you know we're looking for results, and he's given us results every time out."
Upcoming pitching schedule: Enrique Gonzalez has a chance to state his case Monday against the Brewers, while Eveland and Nippert will start Tuesday and Wednesday's games respectively. Owings is slated to start Thursday's game against the Cubs.
Busted slump: Left fielder Eric Byrnes snapped out of a spring slump with a two-homer performance Sunday.
"Sometimes you need to float him around in the lineup a little bit to get his attention again," Melvin said. "He liked being back in that leadoff spot today, and there will be times obviously during the season, especially against left-handers, that he'll be up there to."
Up next: The D-backs play host to the Brewers on Monday with Enrique Gonzalez getting the start. Outfielder Justin Upton, the first overall pick in the 2005 First-Year Player Draft, is scheduled to come over from Minor League camp and be in the lineup in center.
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.