Pascal Wehrlein has emerged as a fourth candidate to become Lance Stroll's teammate at Williams next year.
Earlier, reports circulated that Robert Kubica and Paul di Resta will go head to head in unofficial 'shootout' tests later this month.
Yet Bild quotes Mercedes team boss Toto Wolff as saying: "I think Williams is talking to four drivers.
"[They are] Robert, Paul, Felipe and Pascal."
Kubica's high-profile hopes could get a boost this weekend at Suzuka, with his co-manager Nico Rosberg in the paddock on television commentary duties for broadcaster Sky.
Wolff also referred to German Wehrlein, who is in danger of losing his Sauber seat next year as the Swiss team looks to bring in Ferrari junior Charles Leclerc.
Earlier, it was believed that Mercedes-backed Wehrlein had been ruled out by Williams, as team sponsor Martini wanted at least one 2018 driver to be over the age of 25.
However, Auto Bild reports that Williams have "confirmed that the use of a driver over 25 is not a contractual commitment".
The report explained that at the few countries on the F1 calendar where the legal alcohol consumption age is 25, test driver di Resta could stand in as a PR representative.
