It was an innocuous-looking backhand, a weary service return into the net. And it came more than 90 minutes into the match, although there was still a third set to play.
But when Ana Ivanovic dropped that second set to Simona Halep on Friday evening in Singapore, Ivanovic's season was essentially over. And, at the same time, Serena Williams was given a very fortunate reprieve.
Those are the harrowing lows and soaring highs possible in round-robin tournament play, so ... Saturday's semifinals in the BNP Paribas WTA Finals look like this:
Williams and buddy Caroline Wozniacki will reprise their US Open final encounter, and then Halep will meet Agnieszka Radwanska. The winners play Sunday in the year's final meaningful match.
Heading into Friday's matches, only Halep had qualified for the semifinals. Another score was settled as well; Serena is guaranteed to finish as the WTA's year-end No. 1 for the second consecutive season and Maria Sharapova will be No. 2 for the second time in three years.
It was strange theater, because Serena was nowhere to be seen during the critical match that eventually went to Ivanovic 7-6 (7), 3-6, 6-3.
Instead, there was Sascha Bajin, Serena's hitting partner and all-around handyman, working his phone from a courtside seat. Presumably, he was in contact with his boss, and when Ivanovic collapsed in that second set, Bajin tweeted that he was heading off to book a practice court and added a few smiley faces.
Hundreds would have been appropriate, for Serena should consider herself lucky to still be hanging around. She got blown off the court 6-0, 6-2 in her match against Halep on Wednesday -- her worst loss in more than 16 years. This led to a long talk with coach Patrick Mouratoglou.
"He was like, 'You're still in the tournament. You still have another match to play,'" Serena explained Thursday. "I started to believe that maybe I could come play another match. I wasn't quite sure that I could."
In Serena's next match, she smashed 20-year-old Eugenie Bouchard 6-1, 6-1.
That left her in the precarious position of hoping Halep could take at least a set from Ivanovic. If Ivanovic had won in straight sets, she would have been the second semifinal qualifier from the Red Group (after Halep), based on the percentage of games won.
Give both Halep and Ivanovic credit for battling in a difficult year-end match. Ivanovic trailed 5-2 in the opening set -- and came howling back to win an epic tiebreaker after facing a set point in that frame (at 6-5). And then, after Ivanovic fell into a 4-1 hole in the second, she had some serious chances to get to 4-all. But in the pivotal eighth game, which featured three deuces, she double-faulted twice and looked exhausted during the last few points.
Even with her season done, Ivanovic competed to the end. So did Halep, who may have compromised her chances Saturday against Radwanska. Halep suffered a string of injuries this season and faced Ivanovic with her left thigh heavily wrapped. Playing a 2-hour, 9-minute match into the Singapore night that was, for her, essentially meaningless, was brave but perhaps ultimately unwise.
Earlier, Wozniacki defeated Petra Kvitova 6-2, 6-3 and Radwanska fell to Sharapova 7-5, 6-7 (4), 6-2.
That means the season is over for Kvitova and Sharapova -- the champions at Wimbledon and Roland Garros, respectively -- who each had a 1-2 record in round-robin play in the White Group. Radwanska was also 1-2 in the group, but she managed to advance. In the Red Group, Bouchard (0-3) was the only player not to go 2-1.
Now, what to expect in Saturday's semifinals that can be seen on ESPN3?
Serena, who plays Wozniacki at 2:30 a.m. ET, holds a 9-1 career head-to-head advantage. If it feels familiar, that's because it's their fourth match in 11 weeks. Serena won all three, but two went three sets.
Radwanska and Halep play the second match (6 a.m. ET), and Radwanska holds a 4-2 career edge. They split their two matches this year, with Halep winning in straight sets in Doha and Radwanska returning the favor in March at Indian Wells.