Simona Halep and Agnieszka Radwanska suffered upsets in the fourth round of the French Open before rain brought an early end to the day's proceedings.

Halep came into the tournament having triumphed at the Madrid Open earlier this month but fell to 21st seed Sam Stosur 7-6 (7-0) 6-3 in difficult conditions.

Awaiting the Australian in the quarter-finals is world number 102 Tsvetana Pironkova. The Bulgarian sent the second seed packing by coming from behind to win 2-6 6-3 6-3.

Serena Williams was unable to get onto the court due to the rain delays, meaning the world number one will have to return on Wednesday.

RAIN NOT TO BLAME FOR HALEP

Playing during the rain did not sit well with Halep, but given that Stosur and the Romanian were faced with the same conditions, she felt she had to take responsibility for her elimination.

"I cannot comment about the conditions. I have no words. It was impossible to play, in my opinion, and to play tennis matches during the rain is a bit too much," she said.

"But everyone was in the same situation, and who was stronger won today."

RADWANSKA CREDITS PIRONKOVA

Exactly 100 places separate Radwanska and Pironkova in the world rankings, but the latter won 10 straight games to earn her place in the quarter-finals.

While Radwanska was "p****d" at being made to play amid drizzle, she praised her opponent for taking a care-free approach to the match.

"I think she came on court and she had nothing to lose," said the world number two.

"I was 6-2 and 3-0 up and so I was leading. Well, she just went on court and played her game. She's a very solid player; she did the right things today."

SERENA TO STAY ON TOP

Radwanska's defeat ended her chances of displacing Williams at the top of the world rankings.

The Pole needed to win the Roland Garros title and hope the American failed to reach the final in order to displace her.

TOP HALF FACING FOUR GAMES IN FOUR DAYS

All four of the last-16 clashes in the top half of the draw were postponed until Wednesday due to the conditions in Paris.

If Williams is to mount a successful defence of her title she will be required, as will any of the other players in the top half, to complete a quartet of matches in as many days.

There could be some relief if the final is moved from Saturday to Sunday, while further weather delays could increase the difficulty of claiming the title.