Thomas, 23 next week, has spent four seasons at Victory - he joined from Sheffield United - but has primarily acted as the deputy to Ante Covic and now Nathan Coe.

Since Thomas replaced Coe in the second half a month ago away to Wellington, due to a back injury for which Coe has since undergone season-ending surgery, he has conceded only three goals in just under four-and-a-half matches.

The team has won all but one match during that period, a shock 0-1 home defeat to lowly Newcastle in which Thomas was not to blame for conceding the goal.

Thomas is yet to experience a derby win as the starting goalkeeper. In October 2012 Victory suffered a 1-2 defeat, while in March last year he endured a miserable performance in a 0-4 defeat. He was on the bench for all three of the preceding derbies for this season, of which two were decisively won by Victory and the other a win to City after Erik Paartalu scored the only goal late in the game.

Defensive midfielder Carl Valeri said Thomas, who is out of contract at the end of the season, had impressed since taking over from Coe.

"He's been fantastic, to step in like that. Obviously missing someone like Nathan Coe ... is pretty hard to take, as a player and as a person and as a leader in the team. But 'Loz' has stepped up," Valeri said on Wednesday, before the team trained at Gosch's Paddock ahead of the derby at Etihad Stadium.

"No one has said anything, because he's been here and he trains so hard and works so well we were confident that he was going to do a good job. It wasn't as if 'Oh, Loz is playing, I wonder how he's going to go?'. It was definitely 'Oh, Loz is playing, he's going to do a good job'."

Valeri said there had been "no need" for him to gauge Thomas' mindset as part of the preparations for the derby, but predicted his teammate would be relishing the situation.

"He's such a level-headed person, he's hard-working and he fits into the team perfectly, like everyone at the club. I guess he's excited, just as excited as Archie (Thompson), who's done this many times."

Valeri said preparation devised by Victory's sports science staff would ensure they were not sluggish after earning the right to skip the first week of the finals. He said the players were eager to maintain their regular-season form, having scored the most goals in addition to conceding the fewest.

"Obviously we're very confident after a big season, but in saying that semi-finals are very different ... very special. I think we've had an experienced team whose (members have) been in many battles. We've just got to go out there and do the business. You can talk all you want but we've got to go out there and work as hard as we have worked," he said.

The 30-year-old Socceroo said the opportunity to play in this match, and the grand final should they seal a berth, was why he had returned to Australia after a long career based in Italy, believing the club shared his lofty ambitions.

"Some people say 'Oh, you've won the minor premiership', but it doesn't count for much. We want more, and the first step is beating Melbourne City," he said.

Valeri also said the fact the semi-final was also a derby would not affect his approach.

"You've just got to focus on your job, doing your job for the team and helping your teammates out. I don't think there's time to really take in that atmosphere and everything like that ... personally - and I can only speak for myself - I'm too focused on doing my job for the team and helping my teammates out. It's only the small breaks when the ball goes out that I get to hear the fantastic atmosphere when we score a goal," he said.