Latvia's Anastasija Sevastova ground out a gruelling 6-7(3) 6-4 11-9 win against Elise Mertens to move into the fourth round of the French Open, her best ever showing at the claycourt major.

In a marathon three hour 18 minutes match, the longest contest in the women's draw so far, Sevastova overcame the Belgian to set up a match against Czech Republic's Marketa Vondrousova.

Both players struggled with their service rhythms, with Sevastova producing just 58 percent of first serves in and facing a total of 15 break points. Mertens faced even more.

The 23-year-old Belgian, who arrived in Paris following two first-round defeats in Madrid and Rome, won a comfortable tiebreak to claim the first set. For Sevastova, the errors kept piling up and she frequently turned to her box in anguish.

The Latvian came back strongly in the second set, however, breaking the Mertens serve three times as she found her groove on the backhand.

That set up a monster third set in which the two players slugged it out for 101 minutes, Mertens using her long reach to retrieve seemingly lost points and Sevastova disrupting her opponent's groundstrokes with an array of deft drop shots.

As the contest headed towards its nerve-jangling finale, Sevastova served five times to stay in the match before finally breaking Mertens for a 10-9 lead that left the Belgian totally deflated.

In the end, Sevastova made it look easy, winning the final game to love. 

Karolina Pliskova's quest for a maiden Grand Slam came to an end as the second seed lost 6-3 6-3 to Croatia's Petra Martic.

The powerful Czech struggled throughout to find the range on her feared serve and forehand as 31st seed Martic matched her best performance by reaching the last 16.

Martic broke in the seventh game and then again two games later as Pliskova hit a backhand long to drop the opener.

A lacklustre Pliskova then dropped serve again at the start of the second set before hitting back with a flurry of winners to break her 28-year-old opponent twice in a row.

The fightback did not last though and Matic regained control with two more breaks before serving it out at 5-3.
Martic will play an unseeded player next, either Estonia's Kaia Kanepi or Russian Veronika Kudermetova.

There was better news for Pliskova's fellow Czech Marketa Vondrousova as the 19-year-old reached the last 16 with a 6-4 6-4 defeat of Spanish 28th seed Carla Suarez Navarro.