The Biancoceleste are ninth in the standings, 19 points adrift of second-place Roma yet they appear to be facing their neighbours at the right time.
Since the return of Reja as Lazio coach at the start of January, the club has not lost in Serie A.
Last weekend's 2-0 triumph at Chievo followed their 1-1 draw against Juventus and extended Lazio's unbeaten run to five league games.
"We arrive to the derby with confidence," Reja said. "We played well against Juventus but even better at Chievo.
"It was my team's best performance since I joined Lazio.
"Roma have shown great things this season but we are improving and growing game after game.
"We are not ruling our chances.
"We know the significance of this game and how much it means to our fans.
"My team will try to hand a good performance and take three points."
The atmosphere will be feisty at the Stadio Olimpico with Lazio looking to avenge their 2-0 defeat at Roma last September.
Roma also go into the match in good spirits.
The Giallorossi edged Napoli 3-2 in Rome in Wednesday's Coppa Italia semi-final first leg.
Rudi Garcia's men travel to Naples next week to seal qualification.
"It was important to win against Napoli in view of qualification but also to go into the derby in the best possible way," Roma midfielder Daniele De Rossi said.
Roma have won their last three league games and are nine points behind Serie A leaders Juventus but have a game in hand.
The Giallorossi rested last weekend after their home match against Parma was abandoned after eight minutes due to a waterlogged pitch.
Garcia will have all of his players available for Sunday's match with the exception of injured defenders Federico Balzaretti and Jose Dodo.
Lazio, who are acting as hosts, will be without midfielder Lucas Biglia, through suspension.
Roma captain Francesco Totti will lead the Giallorossi's attack alongside Alessandro Florenzi.