Simona Halep has won two of her previous three meetings with Elise Mertens, in Madrid and Roland Garros in 2018, but was beaten in the pair's most recent encounter, in Doha last year.
Garbiñe Muguruza and Kiki Bertens clash for a sixth time, with the Dutchwoman leading 3-2 in head-to-heads. The duo's last encounter, in Indian Wells last year, went the way of the Spaniard, after three sets.
Anastasia Pavlyuchenkova has beaten Angelique Kerber seven times in their 14 previous matches, but only two of the last six went the way of the German.
Victory in Cincinnati last year gave Anett Kontaveit success in her only previous match against Iga Swiatek.
ORDER OF PLAY
Check out Monday's Order of Play here.
Joel Drucker, of tennis.com, examines the wider implications of Coco Gauff's exit against compatriot Sofia Henin.
Coco Gauff has an eye on the Olympic Games following her Australian Open exit, while Ashleigh Barty is looking forward to reuniting with Petra Kvitova, writes Yahoo.com.
Dominic Hart of The National became the latest to salute the remarkable run of Ons Jabeur.
Cool and breezy, with a mixture of sunshine and showers. Maximum termperature 23C/73F.