Holders Leinster have set up an all-Irish Heineken Cup final after they edged out Clermont Auvergne 19-15 in France.
Another James penalty then moved Clermont narrowly ahead, but Sexton was again on target to make the score 6-6.
In what seemed to be a personal battle between the two players, James booted over two more kicks to give the French side a 12-6 lead at the break.
However, Leinster made an explosive start to the second half as Rob Kearney burst through the Clermont defence before he offloaded to Cian Healy, who touched down. Sexton successfully kicked over the extras to put his team 13-12 in front.
The win was sealed just after the hour as Sexton scored his third penalty of the game to send Leinster through to their third final in four years.
They will now meet Ulster at Twickenham Stadium on Saturday, May 19.