Defending champion Serena Williams overcame her nerves to defeat friend Alize Lim and stay on course for a third-round meeting with sister Venus at the French Open.

While the 6-2 6-1 scoreline looked like a walk in the park for Serena, her 36 unforced errors showed she was not at her best.

The American needed five match points to get over the line after an hour and 18 minutes and knew she had been in a battle.

Serena said: "A lot of the games went to 30-30 or so. So I think the games were actually pretty close.

"It was interesting. I was a little nervous, like I always am in my first round. It's always hard for me to shake those nerves and go from there."

Having been upset in the first round by Virginie Razzano in 2012, Williams won the title at Roland Garros 12 months ago for the first time in 11 years.

She has been arguably more dominant on clay than on any other surface over the past three seasons and came into the tournament on the back of another title in Rome.

"I was in really top form last year on the clay, or at least I thought I was," said Serena.

"This year I'm just going day by day. I do have to say this year I don't feel as much pressure on myself as I did last year."

Lim knows Serena from Patrick Mouratoglou's academy and could not believe the pair were drawn together a day after posing for pictures at the players' party at the Eiffel Tower.

She admitted she had thought about the possibility of losing 6-0 6-0 in her first match on Court Philippe Chatrier.

"Very often that's the kind of match she plays against other girls," said the Frenchwoman. "So I tried to focus on having positive thoughts, and I thought to myself, 'Okay, I have played boys and it can't be worse'.

"She doesn't come from another planet, so if I can move well and I can hit my ball well, there is no reason she should win 6?0 6-0."

Next Serena will play Spaniard Garbine Muguruza then after that a clash against Venus beckons.

The 33-year-old had a tough draw against Swiss Belinda Bencic, virtually half her age and last year's junior champion here and at Wimbledon.

The 17-year-old, who was two months old when Venus played her first French Open, has made rapid strides in the senior game and is already ranked in the top 100.

Her career has been overseen by Melanie Molitor, the mother of Martina Hingis, who was courtside to watch the teenager.

Bencic matched Venus for virtually the whole of the first set but the veteran's extra power eventually told and she ran away with the second to win 6-4 6-1, her first victory at Roland Garros for two years.

Venus, who next meets another teenager in Anna Karolina Schmiedlova, played Bencic for the first time in Luxembourg two years ago when the Swiss was 15.

She said: "I think experience has helped her a lot.

"I think when I played her before, it was really one of her first professional matches, so obviously that's a completely different experience. Even then she played well.

"I think she has a great attitude, and it will take her a long way."

Should the sisters play each other, it would be their earliest ever meeting outside of the round-robin Tour Championships.

Third seed Agnieszka Radwanska had the honour of being the first winner at this year's tournament with a 6-3 6-0 victory over Zhang Shuai of China.

Neither player held serve until Radwanska made it 5-3 in the opening set, but after that the Pole, a quarter-finalist for the first time last year, was untroubled.

Eighth seed Angelique Kerber, 14th seed Carla Suarez Navarro and Daniela Hantuchova are also through.

On Monday, 2012 champion Maria Sharapova begins her campaign against Ksenia Pervak in the opening match on Court Philippe Chatrier while Petra Kvitova and Jelena Jankovic are also in action.