Gael Monfils' hopes of securing a place in the ATP World Tour Finals took a blow with a shock second-round defeat to Gastao Elias at the Stockholm Open.

Frenchman Monfils sits sixth in the rankings and was the top seed in Sweden, but a 7-6 (7-4) 6-1 loss on Thursday cost him the opportunity to cement a spot at next month's tournament at the O2 Arena in London.

Elias staved off three break points in game four and the deadlock continued until a first-set tie-break, when the Portuguese raced into a 4-0 lead.

Despite dropping his serve twice the world number 61 held serve to take the first set, and he quickly moved two breaks up in the second.

Monfils denied Elias on two match points, but the 25-year-old booked his place in the quarter-finals at the next attempt and will face Jack Sock after he dumped out Dustin Brown in three sets.

Elias marked his victory by replicating the celebration of Portuguese football star Cristiano Ronaldo.

"It was a good day. I told someone before the match that I would [celebrate like Ronaldo] if I won," he said in a post-match interview.

"It's the best match I’ve ever played. I knew I could play at this level and compete with the top guys, and this is what I showed today. It’s difficult to play at this level every match, but this is what we practice for."

Second seed Grigor Dimitrov, the 2013 champion, was not posed a particularly stern test by Jurgen Zopp, who he dispatched 6-3 6-2 in 72 minutes.

The Bulgarian will face Kevin Anderson in the last eight, the South African saving two set points before triumphing 7-6 (10-8) 6-4 against Ryan Harrison.

Juan Martin del Potro cruised past Nicolas Almagro 6-4 6-3, with third seed Ivo Karlovic awaiting the wildcard in the quarter-finals, while Tobias Kamke beat Radu Albot 7-6 (10-8) 6-3 to advance.