How does Trent Boult maintain a near-blinding standard over so many years, and across so many formats? Before rain washed out two further sessions of play in Colombo on Friday, Boult became the third New Zealand bowler to 250 Test wickets during a double-wicket maiden that perhaps swung the match in New Zealand's favour. This milestone comes in a year in which he averages less than 22 in Tests, following two full calendar years in which he averaged 23.90 (2018) and 24.77 (2017).

On English tracks, in a completely different format, Boult had also had a good World Cup, having taken 17 wickets to sit eighth on the tournament table. Bowling coach Shane Jurgensen thinks the key to Boult's consistency has been his fitness and his "simple" action.

"Today's a special day for Trent - becoming the third New Zealand bowler to 250 wickets," Jurgensen said. "I think he has a very efficient bowling action. Overall, it's quite simple. I think that allows him to do what he does. Being accurate is really important in all parts of the world. He's got an advantage being left-handed, and that creates variation for our bowling group.

"He's one of the best bowlers in the world and he's proved that for a long time. He's got that skill to expose any type of conditions that are available to him. I also think he can bowl across all different periods of the game. That's really important. He's one of the leaders of being able to do that - the ability to adjust to all periods.

"He's also got great passion and energy which he brings to our team. He's our energiser bunny in the warm-ups. He loves his soccer. I think the skill and execution and the way he's contributed to the team has been amazing. He's a fantastic team man as well. And he does a lot of hard work off the field. He doesn't stop working on his fitness or his resilience. Those factors have contributed to the success he's had."

Also delivering a double-wicket maiden on Friday was Boult's longtime partner-in-crime Tim Southee, who removed Dimuth Karunaratne and Niroshan Dickwella in quick succession. Southee is also only three wickets away from hitting the 250 mark. He is slightly slower to the milestone, however - Boult achieving it in his 63rd Test, while Southee is playing his 67th. On Friday, the pair combined to produce four wickets for just 32 runs.

"Tim and Trent have been doing it for a while," Jurgensen said. "Today there might have been a little bit of swing with the overhead conditions, and it just seemed to be doing a little bit. They both work really hard on their fitness, and that's a big factor in their resilience.

"They're always giving each other a bit of a hard time [because they are so close to each other in terms of career wickets]. That's what's great for the team - they keep things lighthearted. They support each other a lot too - they back each other up on the field and off the field."

Andrew Fidel Fernando is ESPNcricinfo's Sri Lanka correspondent. @afidelf