By Alberto Amalfi | Wednesday, October 21, 2015

Ernests Gulbis has a knack for sudden transformations.

Gulbis arrived in Vienna mired in a five-match losing streak for the third time this year. The enigmatic Latvian embraced his explosive side today.

Video: Serena To Partner Sock in Hopman Cup Return

Gulbis blasted 23 aces toppling third-seeded John Isner, 6-4, 4-6, 6-4, to advance to the Vienna quarterfinals.

The 117th-ranked Gulbis will face either another massive server, seventh-seeded Ivo Karlovic, or Sergiy Stakhovsky for a semifinal spot.

The former world No. 10 played some of his most dynamic tennis of the season. Gulbis hit 54 winners, more than doubling his unforced error output (25), won 10 of 11 trips to net and saved six of seven break points he faced.

In the fourth game, Gulbis dumped successive double faults to stare down double break point. He saved the first with an ace and second cracking a backhand crosscourt. Gulbis erased a third break point with an ace out wide, eventually holding for 2-2.

A bungled drop shot while he was well ahead in the rally put Isner in a double-break point bind. He spit up a double fault deep donating the break and a 4-3 lead to Gulbis.

Gulbis thumped an ace down the middle stamping a love hold for 5-3.

Serving for the set, Gulbis saved a break point with a big serve. A forehand that crashed into the top, popped up in the air and plopped over gave Gulbis a third set point. He closed the set with a serve winner out wide.

The 13th-ranked American answered in the second set. Isner won seven of eight points played on Gulbis' second serve in the second set. When Gulbis bashed a backhand into net, Isner broke to snatch the second set with a shout.

Perhaps relaxing a bit after his set-ending break, Isner immediately fell into a triple break-point hole to open the third set. He would save four break points in the game and appeared to be out of trouble, but Gulbis kept pounding his backhand down the line and on his fifth break point he smacked a searing backhand pass to break for 1-0.

Gulbis never looked back, sealing the victory in two hours, three minutes to reach his first quarterfinal since he fell to Novak Djokovic in the Montreal quarterfinals in August.

Eighth-seeded Fabio Fognini defeated Paul-Henri Mathieu 6-4, 3-6, 6-4, setting up a round of 16 clash with veteran Radek Stepanek.