The British and Irish Lions must execute their plans "immaculately" if they are to emerge victorious from their tour to New Zealand, says Graham Rowntree.
Warren Gatland's players were put through their paces in training for the first time on Monday, with scrum coach Rowntree - himself a two-time Lion - warning of the tough test ahead.
"We want to get the guys used to an intensity of how we train," he said.
"Against the All Blacks the ball is on the field for a long time and we have trained them hard with that in mind. What we do, we have to do it immaculately.
"We have trained them really hard with a double session.
"It's been very useful to get our calls and systems up and running.
"From lineout calls, to phase play, to how we defend, just getting everybody on the same page."
The Lions face a three-Test series across June and July, having been beaten 3-0 on the last trip to face the All Blacks in 2005.
Facing Steve Hansen's world champions on their own turf represents arguably the most daunting challenge in the sport, but Rowntree has been buoyed to see some of the Lions performing well for their club sides.
Saracens, who have six players in the 41-man squad, claimed a second consecutive European Champions Cup title at the weekend with victory over Clermont Auvergne, and Leicester Tigers legend Rowntree was an interested viewer.
"Saracens were very impressive, it's great to see. Back-to-back European champions - that's special that is for any player," he added.
"I thought all our [Saracens] Lions had really big games, that was a game of proper intensity and that is what we want leading into a tour series.
"Those guys getting on the plane after European finals and Premiership finals, that is great."