Robert Helenius has sparred nearly 80 rounds against Anthony Joshua.

Helenius recalled that their sparring sessions were “pretty 50-50,” but the veteran heavyweight is convinced that Joshua will be taken aback by how hard he hits without headgear and much smaller gloves than they used in training camp. England’s Joshua (25-3, 22 KOs) is consistently listed as a 16-1 favorite to beat Helenius, a late replacement for Dillian Whyte, yet the 6-foot-6, 250-pound Helenius is confident he can pull off what would be a huge upset Saturday night at O2 Arena in London.

“I think it’s more pressure on him,” Helenius told BoxingScene.com. “I think he’s probably gonna come in and think he’s overwhelming me or something like that. But I’ve been boxing for 26 years. I’ve been a sparring partner for Joshua. I think we have almost 80 rounds of sparring together. I think he’s gonna be surprised by my punching power.”

The 39-year-old Helenius is also certain that after all the rounds they spent in the ring together nothing about Joshua will surprise him when he squares off with the former IBF/IBO/WBA/WBO champion in a 12-round main event DAZN will stream worldwide. DAZN’s undercard coverage is slated to start at 7 p.m. BST in the United Kingdom and 2 p.m. EDT in the United States.

“It was many years ago, but [our sparring is] still some kind of reference to where we stand,” Helenius said. “And back then, I think it was pretty 50-50, equal sparring. Of course, I’ve been sparring all these big guys in Klitschko camps and Fury camps and Wilder camps. I’ve been all over the world sparring, so I have the experience.”

Helenius hopes Joshua and his team have underestimated him based on his devastating defeat to Deontay Wilder nearly 10 months ago. The hard-hitting Wilder (43-2-1, 42 KOs), a former WBC champion, drilled Helenius with a right hand that knocked him out late in the first round last October 15 at Barclays Center in Brooklyn.

“That’s the best scenario, if they are overlooking me,” Helenius said, “because I think he’s gonna shocked when he steps in the ring with me.”

Finland’s Helenius (32-4, 21 KOs) will fight for the second time in only eight days when he encounters Joshua on Saturday night. Helenius, whose two technical-knockout wins against Adam Kownacki rejuvenated his career, ended a long layoff last Saturday night by stopping Mika Mielonen (6-1, 6 KOs) in the third round of a scheduled eight-rounder at Olavinlinna, a castle in Savonlinna, Finland.

An unforeseen showdown with Joshua was presented to Helenius immediately after he defeated Finland’s Mielonen because the Voluntary Anti-Doping Association revealed “adverse analytical findings” from performance-enhancing drug tests Whyte took during training camp for his scrapped rematch with Joshua. Helenius has a lot of respect for Joshua, but he thinks the timing of their fight is perfect, even on less than one week’s notice.

“He’s a tough fighter,” Helenius said. “He’s always been tough, but I think this is the perfect time to take him for me. … I think that his weak point is that when he gets punched he’s gonna get really nervous and afraid.”

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.