LUGANO, Switzerland - Elise Mertens and Martketa Vondrousova toiled for over two-and-a-half hours through rain drops, but ultimately the No.2 seeded Belgian booked her spot in the quarterfinals of the Samsung Open presented by cornèr on Friday, 6-2, 5-7, 7-5.

With play in the Swiss city affected by rain for a fifth straight day, plans to complete both the second round and quarterfinals on Friday were put on hold, but Mertens led the charge of players who managed to book their places in the last eight.

"With the rain, it's not easy, just to come on, come off court, but it was a really close match and I'm happy I won it in the third," Mertens said after the match. 

"I tried to keep up my game, but of course it was difficult. We were both competing well in the third set."

After winning the first four games of the match en route to rolling to the first set, Mertens was pegged back level by the Czech teenager, following a lengthy rain delay early in the second set, and a back-and-forth affair.

The Belgian led by a break three times in the second set only to see Vondrousova break back each time, before the defending champion, who won this event a year ago when it was held in the Swiss city of Biel/Bienne on indoor hard courts, snuck the fourth break of the middle set to set it to a decider. 

The first to break through in the final set, Mertens served for the match at 5-4 but Vondrousova, not eager to surrender her title, secured a love break as the rain fell on Centre Court.

Nonetheless, the highest-ranked player remaining in the draw steadied the ship, toughed out a pair of multi-deuce games to finish the match, and saved two break points in the final game to advance.

The Belgian World No.20 added: "I managed it well at the end. It started pouring again at 5-4...we wanted to keep playing, and didn't want to go off court again and start again. She's a great competitor and I'm happy that I pulled through in the third."

Mertens' fellow seeds, No.8 Aliaksandra Sasnovich and No.9 Alison van Uytvanck, were not as fortunate in their second round matches, as they were upended by Kirsten Flipkens and Camila Giorgi, respectively.

Sasnovich led Flipkens by a set prior to Friday's first rain delay, and built a double-break advantage upon resumption in the second set, but the Belgian No.3 ran off six of the last seven games to tie the match before building a 5-0 lead in the decider in en route to advancing to her first quarterfinal of the season. 

Giorgi and van Uytvanck played five games in the first set on Thursday before play was suspended, and it was the Italian who had all the answers when the two resumed on Centre Court on Friday.

The World No.59 won six straight games upon resumption to build a 6-2, 3-0 lead, having never lost serve after trailing 2-1 in the opener. 

Two Belarusians will join two Belgians in the quarterfinals, as qualifier Vera Lapko joined Aryna Sabalenka in the final eight with a 6-2, 6-3 win over fellow qualifier Danka Kovinic. 

Germany's Mona Barthel followed up a Thursday upset of No.5 seed Svetlana Kuznetsova with a 6-3, 4-6, 6-4 win over Donna Vekic, while her compatriot, qualifier Tamara Korpatsch, also advanced with a 7-6(2), 6-4 win over Kathinka von Deichmann.

The most dramatic second round victory came for home favorite Stefanie Voegele, who trailed 6-2, 5-3 against No.6 seed Alizé Cornet and saved two match points en route to a 2-6, 7-5, 6-3 win.

“I was two match points down in the second set so I was really lucky to stay in the match. Even in the third set, it was really close. I was 4-1 up but I felt like it could go either way, so I tried to stay focused on every single point," the Swiss said.

“The crowd was supporting me a lot, and it was definitely a big help for me. I really appreciate being here, because the tournament gave me a wildcard, so a big thanks to them and also all the fans that stayed late in the cold and rain."