Paris:Top seed Serena Williams survived a worrying mid-match health scare to defeat Russia's 29th seed Anastasia Pavlyuchenkova 6-1, 1-6, 6-2 and reach the French Open last 16 on Saturday.
The American world number one, who was the 2002 champion here, required treatment courtside after slipping at 0-5 down in the second set, having cruised through the opener.
She had her pulse tested and temperature taken although she did manage to compose herself to win the next game before the 18-year-old Russian went on to level the match.
Williams, still appearing uncomfortable on court, fought off three break points in the third game of the decider.
Pavlyuchenkova, bidding to reach the fourth round of a Grand Slam for the first time, was then immediately broken as Williams unleashed a barrage of heavy forehands to shorten the points as well as her time on court.
The American broke again in the eighth game to take the tie and will face either Israel's Shahar Peer or Marion Bartoli of France for a place in the quarter-finals.
Williams, who won a 12th career Grand Slam singles title in Australia in January, arrived in Paris under an injury cloud after a knee injury sidelined her until May.