WTA Finals: Serena Williams overcomes Ana Ivanovic in opener

Serena Williams
The US Open was Williams' only Grand Slam title in 2014

Serena Williams began the defence of her WTA Finalsexternal-link crown with a hard-fought 6-4 6-4 win over world number seven Ana Ivanovic.

It was the 33-year-old's 16th consecutive win in the tournament, although the Serb gave her a stiff test in this latest encounter in Singapore.

Ivanovic, 26, fought back in the opening set from 4-1 to level at 4-4, before Williams broke again.

The world number one broke once more at 5-4 in the second to seal victory.

Serena Williams's 18 Grand Slam titles
Australian Open (5): 2003, 2005, 2007, 2009 and 2010
French Open (2): 2002 and 2013
Wimbledon (5): 2002, 2003, 2009, 2010 and 2012
US Open (6): 1999, 2002, 2008, 2012, 2013 and 2014

"I'm really excited with how I played. I couldn't be happier to be playing here in Singapore. I was fighting really hard," said Williams.

The end-of-season competition features the top eight ranked players in the world. Two sets of four players face each other in round-robin matches, before the top two in each group advance to the semi-finals.

Both Williams and Ivanovic are in the Red group alongside Simona Halep, who defeated Wimbledon finalist Eugenie Bouchard 6-2 6-3.

The first matches in the White group take place on Tuesday, with world number two Maria Sharapova up against Caroline Wozniacki and Wimbledon champion Petra Kvitova taking on Agnieszka Radwanska.