2016 champion and 2011-12 runner-up Monica Niculescu is one of three past winners in this year's main draw, and has reached more Luxembourg finals than any player since five-time champion Kim Clijsters's last title in 2005. The Romanian qualifier takes on No.4 seed and 2015 semifinalist Alison Van Uytvanck for the fourth time; the pair split two meetings in 2014, with Van Uytvanck winning 7-5, 6-3 in the first round of Wimbledon and Niculescu taking their Beijing qualifying match 6-3, 6-2, but the Belgian took the head-to-head lead three weeks ago with a 6-2, 6-1 victory in the second round of Tashkent en route to her second title of the season.
Both of No.1 seed and 2017 semifinalist Elise Mertens' previous meetings with lucky loser Stefanie Voegele were on indoor hard courts in 2016. Voegele, who made the last four here in 2013 and 2015, took the only completed match 6-1, 6-4 in the first round of the Andrézieux-Bouthéon ITF 50K event en route to winning the title, but retired trailing 6-0, 3-0 in the first round of Luxembourg qualifying.
No.6 seed Camila Giorgi leads 2012 semifinalist Andrea Petkovic 4-2 overall, including a 6-1, 6-2 victory in their sole past indoor match, in the first round of Linz in 2014. That was the third win for Giorgi in three encounters that year, but Petkovic hit back to take the next two in the series at Cincinnati 2015 and Dubai 2016 - before the Italian won their first meeting in over three years, a 3-6, 7-5, 7-6(3) epic in the second round of The Bronx two months ago.
Tashkent runner-up Sorana Cirstea leads the head-to-head against Pauline Parmentier 3-2, including a 6-2, 6-2 victory in their most recent encounter in the second round of Shenzhen in January. However, Parmentier won their only previous indoor match 6-2, 6-1 in the 2016 Croissy-Beaubourg ITF 50K semifinals.
17-year-old qualifier Marta Kostyuk, who reached the third round of the 2018 Australian Open as a 15-year-old and ended last year ranked World No.118, fell to No.320 in April - but has rebounded to her current No.162 since, having compiled a 36-14 record this season that includes the Ukrainian's maiden WTA quarterfinal in Strasbourg and two ITF W60 finals in Torun and Saint-Malo.
An all-German derby finds Tamara Korpatsch taking on qualifier Antonia Lottner for the fifth time. Lottner won their first match 6-0, 6-3, in the second round of the Koeln ITF 10K event in 2013 - but Korpatsch, who reached her maiden WTA semifinal in Lausanne in July, hit back to take the next three in a row, including a 6-1, 4-6, 6-1 victory in Lugano qualifying last year - their only previous encounter at WTA level.
ORDER OF PLAY (all courts start 12 noon)
Laura SIEGEMUND (GER) vs [Q] Marta KOSTYUK (UKR)
 Fiona FERRO (FRA) vs Monica PUIG (PUR)
 Alison VAN UYTVANCK (BEL) vs [Q] Monica NICULESCU (ROU)
Not before 6pm
 Elise MERTENS (BEL) vs [LL] Stefanie VOEGELE (SUI)
Andrea PETKOVIC (GER) vs  Camila GIORGI (ITA)
Pauline PARMENTIER (FRA) vs Sorana CIRSTEA (ROU)
[Q] Antonia LOTTNER (GER) vs Tamara KORPATSCH (GER)
[Q] Chloe PAQUET (FRA) vs  Elena RYBAKINA (KAZ)
Not before 3.30pm
Coco GAUFF (USA) / Catherine MCNALLY (USA) vs Misaki DOI (JPN) / Makoto NINOMIYA (JPN)
 Kristyna PLISKOVA (CZE) / Renata VORACOVA (CZE) vs Fiona FERRO (FRA) / Jil TEICHMANN (SUI)