Wehrlein, 20, was already on track at the Circuit de Catalunya as part of a deal to test for Mercedes-powered Force India, who are running their 2014 car because the new one is not yet finished.
"An unplanned stoppage keeping us off track at the moment. Lewis Hamilton is unwell and not able to continue driving today," Mercedes said on their Twitter feed.
"So we're adjusting the car for Pascal Wehrlein to take to the track this afternoon #musicalchairs."
Force India are attending their first test of the year, after skipping Jerez, and Wehrlein is also due to test for them on Saturday with regular drivers Sergio Perez and Nico Hulkenberg in the car on Friday and Sunday respectively.
There is one more four-day test in Barcelona next week before the season starts in Australia on March 15.
(Reporting by Alan Baldwin; Editing by John O'Brien)
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