SINGAPORE—Serena Williams has been off her game plenty of times before, but the 18-time Grand Slam champion rarely plays horribly. But in her 6-0, 6-2 loss to Simona Halep at the WTA Finals, she never seemed to wake up and, simply put, couldn’t keep her shots in the court.

Without question, the young Halep was very solid, but she didn't have to put together her best strokes. Really, all she had to was keep the ball in play, move it around, and not panic before finishing the match off. That is exactly what the 23-year-old did, scoring her first win over Williams in four career meetings.

Usually, Serena crushes her forehand and lands her strokes close to the lines. But not on this day. Williams dumped shots into the net, was anything but accurate, and couldn’t even get on top of heavy spins. She ended with 36 errors—in just 14 games—the majority of which came from her forehand. On serve, the best women's server of all time only managed just two aces.

When asked about it later, Serena was blunt in her self-assessment. “My forehand was off today again. I guess it went on an early vacation,” she said. “Lord knows my serve was as well. My serve was at best in the 10 and under division in juniors.

“Yeah, it was actually embarrassing, I think, describes the way I played. Yeah, very embarrassing.”

The great Williams was indeed embarrassed early on. Halep came out firing, playing much more aggressive then their last match, in August 2013, when Williams smoked her in Cincinnati, 6-0, 6-4. But Halep has been much better this season, rising to No. 4 in the rankings after reaching the Roland Garros final, where she nearly took down Maria Sharapova, and reaching the Wimbledon semis.

Halep moved very quickly and kept pushing forward throughout the match, hitting deep off both wings. Williams tried to slap her shots back, but was so erratic early on that she dropped an f-bomb by the third game.

In the second set, Williams tried to stay positive and even threw out a “C’mon” after a couple of winners, but she never found her form. Halep won the contest in a rout and showed a bright smile, while Serena was disgusted. And why not? The 33-year-old hasn’t won as few games in a match since 1998, when she went down to Joannette Kruger in Oklahoma City, 6-1, 6-1.

The top-ranked American wasn’t sure if she was going to play in Singapore because of a sore leg. She chose to compete, and defeated Ana Ivanovic in straight sets to open the tournament. But Williams admitted after the loss that she does not feel fantastic. Not even close. “Oh, God no,” she said. “I'm definitely not 100 percent okay. I'm just here playing, but I'm not nowhere near 100% percent.”

Williams praised Halep’s effort, then went further. In fact, Serena said that her opponent had “the best match of her career.”

Williams will face Eugenie Bouchard on Thursday in what could be a difficult physical turnaround. As for Halep, Serena seems to have already put this match behind her. “To be quite frankly honest, I'm looking forward to our next meeting because she is making me going to go home and work hard.”