Sock rocks Zverev
Little more than a fortnight ago, Jack Sock trailed Kyle Edmund 1-5 in the third set of their opening round match at the Paris Masters in Bercy.
Now, the Paris champion finds himself in the semi-finals of the ATP Finals after besting Alexander Zverev 6-4, 1-6, 6-4 in a seesawing encounter that once again demonstrated the American's powers of recovery.
In what was a straight shootout for a spot in the last four, the early exchanges were fiercely competitive, with Sock forced to fend off a break point in his opening service game, while Zverev saved three at 1-1.
After that early burst of drama, both players settled down into some more regulation holds, but it was Sock who was imposing himself on the match, with Zverev struggling to deal with the pace and topspin coming off the American's forehand.
And it was that heavy-duty stroke which delivered Sock the first break of the match at 3-3, a lead which he would consolidate after escaping from 0-40 down.
A battling Zverev saved set point at 3-5, but Sock claimed the opener with a clever dropshot, illustrating that he has much more to his game than just that thunderous forehand.
Indeed, it was Sock's considerable prowess at the net which was causing Zverev problems, but after the German wriggled out of a difficult game to start the second set, he suddenly found himself firmly in the midst of a purple patch, rattling off four games in a row as he began to find the sweet spot on his racket with increasing regularity.
Sock stopped the rot by holding for 5-1, but Zverev quickly wrapped up a 28-minute second set to force a decider.
The momentum looked to be firmly with the German, not least when he broke at the first time of asking in the third set, expertly withstanding pressure from the Sock forehand before drawing the error.
A furious Sock responded by launching a ball into the crowd in frustration, incurring a point penalty in the process, and one had to wonder whether his race was run.
But just as quickly as it had appeared, Zverev's rich vein of form vanished, with the world No.3 producing two double faults and an error to tamely relinquish his serve and hand the initiative back to his opponent.
I'm playing with house money. I'm going out there and letting loose
That sudden swing in momentum breathed new life into Sock, who raced to a 4-1 lead, aided by more double faults from the Zverev racket at 2-1.
However, another twist was forthcoming as Zverev pegged his opponent back to 4-4, putting the match firmly back in the balance. But once again, Sock came good in the pressure moments, finding some big serves to hold for 5-4 before drawing an error from the Zverev forehand to claim victory and move into the semi-finals.
"I'm playing with house money. I'm going out there and letting loose, having fun, trying to put on a show a little bit, just enjoy my time,” said Sock.
"If I put pressure on myself, I probably wouldn't be here in the first place. I think getting through that first match in Paris, playing Edmund, honestly I should have lost, but ended up winning the tournament.
With that attitude, and in this kind of form, Sock will likely pose a stern test for Grigor Dimitrov in the semi-finals. Zverev, meanwhile, sees a breakthrough season which yielded five titles and the world No.3 ranking come to an end in a frustrating manner.
"I choked. It's quite easy," said the German.
"Won the second set 6-1. I was 1-0 with a break. He got a point penalty. I was down 1-4 within 10 minutes where I didn't put many balls in the court. So, yeah, I just choked."
Federer stays perfect
Roger Federer advanced to the last four of the ATP Finals unbeaten for the 10th time in his career after coming from behind to beat Marin Cilic 6-7(5), 6-4, 6-1.
In direct contrast to their previous meeting in the Wimbledon singles final, the pressure was off both players, with the Swiss already confirmed as group winner and Cilic eliminated from semi-final contention.
But it was the Croatian who seemed to benefit most from the circumstances, visibly growing in confidence after saving three break points in the opening game.
Federer, meanwhile, was struggling for his best tennis, and saw the opening set slip away after Cilic lashed a forehand winner past him at 6-5 in the tie-break.
Cilic, who was serving big and swinging freely, continued to respond well to break points in the second set, frustrating Federer in his efforts to get the match back on level terms.
Unfortunately for the Croatian, though, he cracked when serving to stay in the second set at 4-5, dragging a backhand wide to set up a decider.
There, Federer clicked into gear, breaking Cilic's serve at the first time of asking after dispatching a tame second serve with a rasping forehand winner.
Now in something approaching cruise control, the Swiss broke his opponent once more before serving it out to move into the semi-finals with a blemish-free record from the round robin stage.
"I think in the beginning I was slightly overaggressive in the first set on some crucial points rather than maybe being patient," said Federer after the match.
"I was able to turn that around late in the second set. Then just had a much better feeling in the third set.
“Maybe also Marin's level dropped, I'm not sure. But definitely was dangerous there for a while."
Federer will now face either David Goffin or Dominic Thiem in the semi-finals, while Cilic heads home without a win to his name - although he was remaining positive.
"Even though it's three losses, I felt that my tennis wasn't too far off of playing really, really well," he said.
"Good positives at least in that. Hopefully I'm going to recover good and start a new season on a good level."
Jack Sock giving the 'videobomb' perhaps its debut outing at the ATP Finals...
WATCH: Roger Federer was analysing his third straight win at the ATP Finals in London when he was surprised by bobble-wearing Kansas City Chiefs fan Jack Sock at the Sky Pad on Thursday: https://t.co/NPflVuD79f pic.twitter.com/25ztMzV1GR— Sky Sports Tennis (@SkySportsTennis) November 17, 2017