Juan Martin del Potro closed in on a World Tour Finals place by beating Robin Haase 7-5, 6-4 in the Paris Masters last-16 on Thursday.
Krajinovic stunned Isner with a thrilling 6-4, 6-7 (2/7), 7-6 (7/5) victory to reach the Paris Masters final.
Filip broke him just once, from a lone break point he gained, and the Serb fends off 5 break points to keep his serve intact throughout the match, showing clutch and determination in the crucial moments to edge the American and continue his Paris dream.
Krajinovic, who dropped out of the world's top 200 past year after surgery to remove an extra bone in his right wrist, dug deep to come through a deciding-set tie-break against the big-serving Isner, who struck 31 aces. It was tough to control the emotions, but I knew I had to stay aggressive all the way to the end. "It was mentally really tough because, you know, getting older and older all sponsors said no to me".
The popular Argentine, who was at No. 47 in the Emirates ATP Race To London prior to the start of the US Open, is now up to No. 10 on 2,595 points - only 20 points behind Spaniard Pablo Carreno Busta, who sits in the last automatic qualification berth.
I had a look at handicap lines, but I wasn't particularly enthused by the 11/10 about Krajinovic +2.5 games with the Sportsbook - I'd prefer around 5/4 - and likewise Krajinovic +1.5 sets at around evens.
Isner went on to win the set on a tie-break to take it into a decider.
The 77th-ranked Krajinovic had only won 17 matches in his whole career - thwarted by a wrist injury - before this tournament. Having seen Krajinovic take out two top Americans, Jack will not want to become the third in a week.
Sock, 25, saw off a spirited Benneteau with little trouble to deny the Paris crowd a first home finalist since Jo-Wilfried Tsonga six years ago and close in on London.
In a tense decisive tiebreaker, Isner led 3-0 but Krajinovic rallied to move 5-4 ahead.
However he succumbed in three sets despite putting up a massive fight in the second to level the match.
'He said I'm sorry to me 10 times, like a kid who's done something wrong.
Everything could have been much different if John converted any of the 3 break points he had in the opening return game, but Filip repelled them all with some fine groundstrokes, especially from the backhand side, to avoid an early setback.
The 83rd-ranked Benneteau was struggling against Sock's powerful forehand and fell a break down in game five of the second set.