Ireland captain Rory Best believes Johnny Sexton is in incredible form after the number 10's long-term injury.
Ireland handed England their first defeat since 2015 after an inspired performance from Sexton after the fly-half scored eight of his side's 13 points.
Best believes Sexton's competitive attitude helped him gain his form back.
"The guy's a competitor. Probably the best thing you can say is that I definitely prefer playing with him in the green of Ireland than playing against him when he's with Leinster," Best told reporters.
"He's a bit of a nightmare to play against and he just has this real will to win. There's a few out-halves in world rugby now that have it, especially in the northern hemisphere.
"But as much as anything what we see on the pitch, that competitor that he is, that's what you need to win games and that's all that is.
Best went onto to praise Sexton's humility of the field.
"Johnny off the pitch is a really, really good guy. The way Paddy Jackson learns off him and the way the team feeds off him, it's great.
"He's a real warrior on the pitch but a really humble, good guy off it."