02/14/2013 02:01 EST | Updated 04/15/2013 05:12 EDT

Isner powers past Pospisil in 3 sets to move into SAP Open quarter-finals

SAN JOSE, Calif. - The nagging knee that kept John Isner out of the Australian Open last month is no longer bothering him. Instead, the six-foot-nine American has a bad back from sitting in a cramped economy class seat on two cross-country flights.

With storms cancelling his original plans, Isner was forced to travel from Jacksonville to San Jose via Houston this week. He sat by the window for both flights, unable to get any extra-leg space.

"Sure enough there was some like 5-foot-1 person with their feet dangling on the exit row," Isner said.

The top American is still getting comfortable on the court again, too.

Isner powered his way into the quarter-finals of the SAP Open on Wednesday night, pounding 14 aces in a 7-6 (3), 2-6, 6-3 victory over Vancouver's Vasek Pospisil.

The hard-serving American overcame a shaky second set, held off two break points in the third set with aces and finished off the pesky Pospisil behind some pretty passing shots. Isner will play the winner of Xavier Malisse and Matthew Ebden on Friday.

"Very, very glad to be sitting here a winner and not a loser thinking about my economy seat to Memphis tomorrow," Isner said, smirking.

After dropping out of the year's first Grand Slam and losing his final Davis Cup match in the United States' win over Brazil last weekend, Isner hardly showed the poise and polish of his past.

He still hit 77 per cent of his first serves, which topped out at 137 mph, winning 77 per cent of those points to make up for most of his misses. Isner, ranked 16th in the world, mixed up his speeds in the final set once Pospisil started to solve his serve.

Pospisil still won more overall points (87) than Isner (86) and had the American on the verge of elimination. With Pospisil ahead 2-1 in the final set, Isner blasted aces on both break points he faced and smashed another serve that went unreturned to level the match.

"My serve bailed me out like it has a lot of times," Isner said.

In the next game, Pospisil pushed a volley wide and hit a backhand long to go down 0-40. After Pospisil saved one break point, Isner bent down low for a big-finish forehand winner down the line that had him charging the net, pumping his fist and yelling, "Yeah!"

While it's not the clean West Coast comeback Isner had hoped, the ending is better than his recent matches.

Isner lost to Thomaz Bellucci to send the U.S. and Brazil to a deciding fifth match Sunday, when Sam Querrey sealed the victory for the Americans by beating Thiago Alves indoors in Jacksonville, Fla. Isner also withdrew from the Australian Open because of a bone bruise in his right knee.

Since losing in the round of 32 in the U.S. Open last August, Isner entered San Jose just 3-7.

That's quite a slump considering he had a year-end best No. 14 world ranking and a career-high $1,354,332 in earnings in 2012. He hadn't played on the ATP World Tour since a two-set loss to fellow American Ryan Harrison in the second round in Sydney on Jan. 9, and he's hoping to regain his rhythm before the summer season.

Most of the match still played out in typical Isner fashion: few rallies, fewer returns and even fewer service breaks. About the only thing in abundance was frustration by his opponent, who seemed to see Isner's serve well, but just not well enough.

In the first-set tiebreaker, Isner earned a minibreak in an instant. The anything-but-nimble-giant reacted rapidly to a body shot at the net, returning it for a winner to go ahead 4-2 that had Pospisil pedalling back to the baseline exasperated.

That's all Isner needed. Pospisil never returned one of Isner's serves in the tiebreaker, and his backhand landed wide to give Isner the tiebreaker at 7-3.

Pospisil persevered by taking advantage of a double fault and made Isner play from the baseline for a break at 4-2 in the second. Isner seemed to conserve his energy and was broken again, leaving most of the announced crowd of 3,163 for the night session inside the home of the NHL's San Jose Sharks stunned and silent.

That didn't last.

After tying the match at two games each in the final set, Isner never looked lost again. Breaking Pospisil's serve at 3-2 deflated his opponent, who sailed a backhand on match point to drop another service game.

"I play a lot of three-set matches," Isner said. "I rarely win matches easily."

Isner isn't the only strong server in San Jose this week.

Two-time defending SAP Open champion Milos Raonic of Thornhill, Ont., is the top seed in the draw, setting up the possibility of a power-packed final should both advance. The 13th-ranked Raonic also had an opening-round bye. The Canadian will play in the second round Thursday night against Michael Russell, who won an All-American matchup over qualifier Donald Young 6-3, 7-5.

Also Wednesday, Colombia's Alejandro Falla outlasted Flavio Cipolla of Italy 7-6 (1), 7-6 (10); Australian Matthew Ebden beat qualifier Rik de Voest 6-2, 7-5; wild-card Steve Johnson upset Croatia's Ivo Karlovic 6-7 (2), 6-4, 7-6 (6); and fourth-seeded Tommy Haas of Germany beat Ottawa's Jesse Levine 6-3, 6-1 in the late match.