Oakland Raiders head coach Jon Gruden might not concede much, but when he thinks back 26 years when he joined the Green Bay Packers coaching staff as an energetic and driven young coach who treated film study more vital than sleep, he couldn't help yield to the overwhelming work ethic of one of his contemporaries — Andy Reid.

Reid, now in his 20th season as a head coach in the NFL and sixth with Chiefs, and Gruden, in the midst of his 12th season and second stint with the Raiders, were just a pair of offensive assistants still cutting their teeth in the professional ranks when they joined Mike Holmgren's staff with the Packers.

Sunday in Oakland, they'll tangle as head coaches of rival franchises for the first time since 2006, when Reid coached the Eagles and Gruden the Tampa bay Buccaneers.

"I kind of take pride in being one of the all-time grinders," Gruden said, reflecting on their time together in Green Bay during a conference call this week. "I don't lead the league in anything, but I always try to lead the league in effort — at least getting up early, staying late, working hard at it. Reid beat me to the office every day, and he stayed later. He loves football more than me. He's one of the few guys I know in my life that actually like football more than me.

"That's what I love about him and admire about him the most, he's been able to maintain his passion. He's been able to maintain his leadership. It's really a credit to him and a big part of why the Chiefs and the Eagles and wherever he's been have been successful."

The two have maintained their mutual admiration, evident by the fact that Reid felt comfortable opening up the doors and allowing Gruden to sniff around the facility as a media member and broadcaster last year during the playoffs even after Gruden had agreed to take the coaching job with the division-rival Raiders.

"We know each other, I mean I trust him," Reid said. "He wasn't going to do anything that I worried about. He wasn't going to sneak around or anything. That's not how he rolls. I really wasn't worried about it. I know how much he studied every week what we'd been doing as he did the other teams. We weren't hiding — I wasn't hiding anything from him."

Twenty-six years after combining forces with the Packers, the duo has combined to win more than 300 games (regular season and postseason), 13 division titles, two conference titles and a Super Bowl win courtesy of Gruden's Tampa Bay Buccaneers. That's with Gruden taking 10 years away from the field to be a full-time television analyst for ESPN and part-time pitchman.

Reid's Chiefs (9-2) have the best record in the AFC and feature an MVP candidate in first-year starting quarterback Patrick Mahomes. Earlier this season, Reid reached the 200-win plateau as an NFL head coach including a 14-season tenure at the helm of the Philadelphia Eagles.

On the other hand, this season marks Gruden's return to the sideline for the first time since the 2008 season with the Buccaneers. He spent seven seasons with the Buccaneers following a four-year run with the Raiders (1998-2001). Before becoming a head coach, Gruden served as offensive coordinator for the Eagles from 1995-1997 (before Reid's arrival in 1999).

"He's a phenomenal football coach," Reid said. "He loves it. He's passionate about it. He was passionate about it when he was broadcasting. He didn't get too far away from the game. He was right there. He studied like crazy, just like he was still coaching. His offseasons were spent that same way, studying the game. The one thing he was able to do was he got to study everybody and visit and see how things were done at other places. So when people said it would be tough for him to come back, I knew how close he had remained to game."

Gruden's return to coaching has gotten off to a lackluster start so far. His Raiders have gone 2-9, and he and the front office have been widely criticized for trading away one of the league's most dynamic edge rushers and 2016 Defensive Player of the Year in Khalil Mack following a contract holdout during training camp.

The Raiders also traded wide receiver Amari Cooper mid-season, and he's already matched or surpassed his production from the first six games with Oakland in just four games with the Dallas Cowboys.

"Listen, he's doing it his way, and I think you're going to see that over the year," Reid said. "The way he's going about it, it's not going to be a change overnight. It's going to be something that takes a little time. He understands that. He's trying to build that to where it lasts and it last for a long time."

Gruden has had success against Reid in the battles as head coaches. Gruden's teams recorded victories against Reid's Eagles in 2001 (Raiders) and 2002, 2003 and 2006 (Buccaneers). Gruden's Buccaneers earned an appearance in the Super Bowl by defeating Reid's Eagles 27-10 in the NFC title game in 2002 after the Eagles won the regular-season meeting. The Buccaneers went on to win the Super Bowl.

"I know what we're in for," Gruden said when asked about his track record against Reid. "I know what his record is. I know what the Chiefs are right now. I know what they're capable of doing. I know what the point spread is. I know a lot about what I see on tape. I don't care about what has happened in the past.

"I know Andy Reid coming off a bye week is a problem. This Chiefs team that I'm studying is really, really good. I'm sure they're really fresh. I'm sure he's managed the bye like he always does. We're going to get their best shot, and we've got to be ready."

Reid's teams have compiled a 16-3 record in games coming off of a bye week as will be the case this weekend.