Tennis player Kei Nishikori spotted wearing a Casio

Kei Nishikori was photographed on court wearing what at first glance appears to be a standard-issue Casio G-Shock GA-2100 — until the rainbow-indexed dial gives it away. The watch is a bespoke collaboration piece produced by UK-based retailer IFL Watches in partnership with The Dial Artist, a customisation studio that has built a significant following for its hand-painted, re-dialled interpretations of mass-market sport watches. The grey colourway and multicolour hour markers are the signature of this limited run.

Under the octagonal resin case, the movement is Casio's own 3459 module — a dual-layer analogue-digital calibre offering world time, five daily alarms, a stopwatch, and 20-bar water resistance. The GA-2100 case architecture, launched in 2019, drew immediate comparisons to the Royal Oak's octagonal bezel geometry, earning it the 'Casioak' nickname from the watch community almost overnight. It measures 45.4 mm across but wears considerably smaller thanks to a slim 11.8 mm profile, making it genuinely wrist-friendly for athletic use.

What elevates this particular example beyond the standard GA-2100-8A is entirely the aftermarket work. The Dial Artist strips, re-paints, and reassembles each unit individually, applying multicolour luminous-style indices, a matching grey bezel with colour-coded button labels, and a coordinated strap — transforming a sub-£100 watch into a conversation piece priced at £570 ($660). Each unit is effectively a one-of-a-kind object, which gives the collaboration genuine collector currency despite the humble base calibre.

Nishikori's affinity for Japanese brands is well documented — he has worn Seiko on various occasions — but the Casioak choice is more personal than patriotic. As a Uniqlo ambassador who has long embraced functional, design-forward Japanese aesthetics, the grey Rainbow Casioak fits his on-court identity precisely: technical, understated in silhouette, bold in detail.

On the secondary market, completed IFL x Dial Artist Casioak customs trade between $500 and $750 depending on colourway and condition, broadly in line with the original retail. Unlike most grey-market watches, these rarely depreciate sharply — scarcity and the hand-made nature of each piece underpin values consistently. For collectors at the intersection of street culture and horology, Nishikori's wrist time has only added to the model's profile.