Serena Williams suffered her earliest grand slam defeat for five years with a third-round loss against fellow American Sofia Kenin at the French Open.

Not since a defeat in the same round against Alize Cornet at Wimbledon in 2014 has Williams been beaten before the quarter-finals at one of the sport's biggest events.

Twenty-year-old Kenin may be a new name to many fans but she has had a very good year so far and refused to be overwhelmed by Williams' presence in a 6-2 7-5 victory.

While it is a big surprise on paper, Williams arrived in Paris desperately short of matches and with her fitness in doubt because of a left knee problem.

The 37-year-old had not played two full matches in any tournament since the Australian Open in January and Kenin was simply far sharper and quicker around the court.

Kenin was groomed for stardom from a young age and the most impressive thing about this victory was the way she matched Williams for competitiveness.

After breaking the Williams serve twice, she thought she had served out the opening set and was almost back to her chair when she realised her serve had been called wide.

Other players might have let that affect them but Kenin refocused, saved a break point and clinched the set anyway.

Williams lost the first set of her opening match against Vitalia Diatchenko and then won all but one game, but Kenin was a higher class of opponent, and she broke her opponent's serve to love to open the second set.

Williams looked slow in thought and action but the 23-time grand slam singles champion is nothing if not a fighter and she dug her heels in just in time.

Kenin faced the full effect of Williams' stare as she tried, and failed, to hold serve for 4-2, and soon she was taking on the crowd as well, who were unhappy with her eagerness to check close line calls.

But Kenin appeared unflustered by it all and another dip in Williams' level in the 11th game left Kenin serving for the win of her young life.

She netted a forehand on her first match point but again kept her composure and a backhand long from Williams sent Kenin through to the fourth round of a slam for the first time.