SINGAPORE - Serena Williams won just two games Wednesday as she crashed to her heaviest defeat in 16 years and raised concerns over a knee injury that could dash her hopes at the WTA Finals.

The 18-time Grand Slam champion was a shadow of her dominant best as she slumped 6-0, 6-2 to Romania's Simona Halep, her biggest loss since she was 16 and without a tournament win to her name. The seismic defeat ended a run of 16 straight victories at the year-ending tournament for the 2001, 2009, 2012 and 2013 champion, whose bid for a title hat-trick is now hanging in the balance. Elsewhere in the Red Group round robin, Ana Ivanovic maintained her semi-final hopes with a 6-1, 6-3 win over an out-of-sorts Eugenie Bouchard.

"My forehand was off today again -- I guess it went on an early vacation," Williams said. "Lord knows my serve was as well. "My serve was at best in the 10-and-under division in juniors," she added. "Yeah, it was actually embarrassing I think describes the way I played. Yeah, very embarrassing." Given the high profile of the season finale in Singapore, it was a career low for the 33-year-old who had not been beaten so soundly since losing 6-1, 6-1 to Joannette Kruger in Oklahoma City in 1998.

She signalled that she was a long way from recovering from the left knee injury which forced her to withdraw from the China Open in Beijing earlier this month. "Oh, God no. I'm definitely not 100 percent okay. I'm just here playing, but I'm not nowhere near 100," she said, when asked about her injury. "That has nothing to do with today's match. I think Simona played really well and the best match of her career." Williams will now have to fight to reach the semi-finals in her final round robin match against Bouchard, with Red Group potentially coming down to calculations of sets and games won.

Halep, 23, said she was now confident of reaching the last four after pulling off a psychologically important victory -- her debut win over Williams, and her first against a world number one. "Yes, I think it was my best match in my life," said the world number four, the first Romanian to qualify for the tournament since 1998. "I'm a little bit surprised, but I knew before the match that I have nothing to lose, just play. She's the best player in the world, she's number one. "It's the last tournament of this year. I had injuries. I had a tough period. Now I'm really happy and I really believe in myself."

The diminutive Romanian exploited some horrendous serving by Williams, who threw down seven double faults and won just 29 percent of points on her second serve, and was broken five times. Halep was already up 2-0 in the second set before Williams, 10 years her senior and the oldest player in Singapore, finally made it on to the scoreboard with her first hold of serve. And Halep, who had never beaten a top-three player before, saved six break points in the second set as she resisted Williams' efforts to turn the match around in front of a shell-shocked crowd.

The first set was a rare "bagel" for Williams, who last lost a set 6-0 against Anabel Medina Garrigues at Madrid in 2013 -- but went on to win the match anyway. Later, Serbia's Ivanovic threatened to inflict similar punishment on Bouchard before the Canadian world number five finally won a game to go 1-5 in the first set. The second set was tighter after they exchanged breaks in the opening two games but Ivanovic broke again to go 4-3 ahead, and a third time to take the match when Bouchard put a forehand long. After the second round of matches, Halep was top of Red Group with two wins, followed by Williams and Ivanovic on one win and one loss, with Bouchard bottom.