Marin Cilic will not seal an ATP Finals spot this week after suffering a shock second-round defeat to qualifier Marius Copil at the Swiss Indoors Basel.

Cilic would have been assured of a place in the season-ending tournament in London next month by winning the title in Basel, but lost 7-5 7-6 (7-2) on Wednesday.

A struggling Cilic had an abysmal first-serve percentage of only 39 and failed to secure a solitary break as world number 93 Copil pulled off a big upset.

Cilic made early exits at the Shanghai Masters and Japan Open Tennis Championships after losing in the quarter-finals of the US Open and Copil capitalised on the Croatian's loss of form, firing down 21 aces as he set up a quarter-final against Taylor Fritz.

Second seed Alexander Zverev became the first player to take his tally of victories for the season to 50 with a 6-4 7-5 first-round success against Robin Haase.

Stefanos Tsitsipas, winner of his maiden ATP World Tour title in Stockholm last weekend, saw off Jeremy Chardy 6-2 7-6 (7-3), while Ernests Gulbis ousted fifth seed Jack Sock to join Roberto Bautista Agut and Dusan Lajovic in the round of 16.

Kevin Anderson remains on course to feature in the ATP Finals after he made it through to the quarter-finals of the Vienna Open without playing when Jurgen Melzer withdrew.

Grigor Dimitrov's slump continued in the Austrian capital, where Mikhail Kukushkin consigned the third seed to a 6-4 4-6 6-4 first-round defeat.

Kyle Edmund, another first-time ATP Tour champion on Sunday, defeated Diego Schwartzman 6-3 7-6 (7-3), while Borna Coric moved into the last eight along with Marton Fucsovics.

Gael Monfils and Karen Khachanov moved into the second round.