(CNN) -- It's been that type of year for Serena Williams.
Williams collected her 18th grand slam singles title at the U.S. Open in September to match retired legends Martina Navratilova and Chris Evert but prior to that didn't get past the fourth round at the three other majors in 2014.
The world No. 1 recently retired due to illness in Wuhan, China and then was a walkover in Beijing with a knee injury -- and on Wednesday at the WTA Finals in Singapore she suffered the joint worst loss of her career.
Simona Halep battered Williams 6-0 6-2 to leave the two-time defending champion in danger of being eliminated in the group stage.
It matched the number of games won by Williams in 1998 -- when she was just 16 -- against South Africa's Joannette Kruger in Oklahoma.
Williams, two days after looking sharp in defeating Ana Ivanovic, made 36 unforced errors in the 14 games and the encounter lasted a mere one hour and five minutes.
While Halep went 5-for-6 on break points, Williams went 0-for-6.
"Embarrassed is the way I would describe how I played," Williams told reporters. "Definitely embarrassing."
"My forehand was off today. It's obviously gone on an early vacation. My serve was at best at the 10-and-under division of juniors."
Halep improved to 2-0 in the Red Group.
The Romanian -- one of two debutants at the WTA Finals along with Canada's Eugenie Bouchard -- entered the year-end championships in patchy form but she limited her own unforced errors and covered the court with aplomb.
Halep had lost both her matches to Williams last year, with the latter inflicting two "bagels" -- or 6-0 sets -- of her own.
"It was my best match of my life," Halep told reporters. "I played unbelievable shots. At an important time I played really well and after today my confidence will be more high."
Ivanovic eased past a listless Bouchard 6-1 6-3 later Wednesday. Both players hit 20 winners but Bouchard committed 31 unforced errors compared to the Serb's 18.
Caroline Wozniacki and her pal Agnieszka Radwanska posted upset wins Tuesday over Maria Sharapova and Petra Kvitova, respectively.