But the hosts - who finished seventh in the table to secure the European play-off spot - rallied after the break, recovering from 24-0 down to finish within four points of their opponents, crucially denying them another score.
That result means the Ospreys will travel to Munster in the final four, although centre John feels an away tie will alter nothing about their preparation.
"The semi-final is huge wherever you play it, being at the Liberty Stadium wouldn't have guaranteed anything so the task doesn't change," the 24-year-old told the Ospreys website.
"We've got to win, and beat a very good team, so we know what we have to do.
"It's just about getting the win - there's no bonus point, there's no tactical play, it's about performing as best you can and hopefully get the win at the end of it.
"That's what play-off rugby is all about, 80 minutes and winner takes all. We can forget the last 22 matches now, it's all about what happens at Thomond Park and then, if we do the job, at the Kingspan Stadium a week later.
"The win was the number one thing when we went to Connacht and we held on and that's important.
"Even if we missed out on the bonus point, we've still got momentum building and we are heading in the right direction.
"We have to build on that now, get a good week's prep in and then make sure we perform to the best of our abilities next Saturday."
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