Roger Federer insists he has no regrets despite missing out on finishing the year as world number one.
Federer's decision to skip the Paris Masters last month ended any hopes he may have had of catching Rafael Nadal at the top of the rankings.
Nadal subsequently withdrew from this week's ATP Finals in London after just one match through injury, clearing a path for Federer to claim a seventh title and the potential 1500 ranking points on offer.
Yet victory in the final on Sunday would leave the 36-year-old Swiss superstar just 140 points shy of his great Spanish rival.
However, Federer said: "Regrets? I don't have any. But maybe losing to Tommy Haas (in Stuttgart) and Evgeny Donskoy (in Dubai) ended up haunting me.
"I had match points in both matches. It's not like I didn't try.
"To have regrets because of these matches - if I miss out because of that for world number one, then maybe I was unlucky.
"But I also did win matches in Miami, saving match points against Tomas Berdych, and other matches throughout the season could have gone either way as well.
"So I'm just happy I'm playing a great season. I'm so happy that I was able to reach this level of play and still being able to play also at the end of the year.
"I have no regrets because I totally exceeded my expectations. I'm just happy that I'm injury-free and healthy right now and enjoying myself still."
Federer did not appear to be enjoying himself a great deal after finding himself a set behind and facing a break point against already-eliminated Marin Cilic in the final round-robin game.
Having qualified for the semi-finals regardless of the result, the 19-time Grand Slam winner could have been forgiven for taking a rare defeat on the chin and saving himself for Saturday's meeting with either Dominic Thiem or David Goffin.
Yet, ever the perfectionist, Federer dug deep to run out a 6-7 (5/7) 6-4 6-1 winner ahead of his 14th semi-final in 15 appearances at the Finals.
Cilic, who endured a painful defeat to Federer in July's Wimbledon final a few miles along the Thames, said: "I think today was a solid match.
"Obviously it's a little bit more difficult mentally knowing that I'd had two losses, and that I wasn't going to be able to go through.
"But still I played those first two sets at a really good level. I even had a break point there early in the second.
"In the third set I felt like I dropped a little bit with my serving percentage. Once Roger got the break, he was playing little bit looser. He obviously played well."
Later on Thursday Jack Sock won a see-saw encounter with Alexander Zverev to secure a semi-final against Bulgarian Grigor Dimitrov.
The American was well on top as he took the first set against third-ranked Zverev, but a major shift in momentum saw the 20-year-old German run away with the second.
Zverev took an early break in the third, prompting Sock to blast a ball into the crowd earning a penalty point from the umpire.
Yet Sock took the next four games, only to be pegged back to 4-4, before a double fault from Zverev offered up match point and a wide forehand saw the eighth seed through 6-4 1-6 6-4.
"That was a roller-coaster match for sure," said Sock.
"It's kind of been my theme in the last few weeks to hang in there. Sometimes you have to let your frustration out and it worked I guess.
"Dimitrov has played some incredible tennis and had an amazing 2017 so I will have to play some of my best tennis to win."