“I feel confident with my shots but I need to improve a little bit,” the eighth-ranked Argentine said. “I would love to be in the final, but I need to serve well and find a way to break [Muller’s] serve.”
Del Potro saved a break point in his opening service game against Matosevic and two more while serving for victory, but was in control throughout the match.
Muller advanced after defeating Paolo Lorenzi of Italy 6-3, 6-4. Muller had 10 aces and earned one break in each set to reach his second semi-final of the year.
Tipsarevic’s match took just over an hour. Bedene, playing in his first career quarter-final, won the first nine points on his serve in the opening set but lost the next four as he was broken at 2-2. Tipsarevic dominated from then on as Bedene became hampered by his injury and struggled to move.
“I would rather win in a normal way but it’s good for me this time,” the ninth-ranked Serb said. “I had a doubles match late last night and didn’t get to bed until half past one.”
Tipsarevic’s chances of qualifying for next month’s season-ending ATP World Tour Finals in London would be boosted by a good result in Vienna.
“I am ready, I am mentally fresh, I am hungry,” Tipsarevic said. “I still feel there is room for improvement in my tennis. There are small things that you can add to your arsenal of weapons.”
Later on Friday, the 70th-ranked Zemlja broke Haas’ serve at 2-2 in the deciding set and did not drop a point afterward to reach his first career semi-final.
“This is a huge surprise, I didn’t even expect to win my first round here,” said Zemlja, who had to qualify for the main draw and who was 0-5 against top-20 players.
“I played well. Now I have the confidence that I can beat the top players,” he said.
Top-seed Jo-Wilfried Tsonga defeated Sergiy Stakhovsky 6-3, 6-4 on Friday to earn a place in the Stockholm Open semi-finals against 2009 champion Marcos Baghdatis.
The seventh-ranked Frenchman, playing in the Swedish capital for the first time, put on an impressive display of serve and volley tennis to advance.
Leading 5-3 in the second set, Tsonga squandered four match points on Stakhovsky’s serve. However, on his own serve, Tsonga finished with a powerful forehand winner on the fifth match point.
Cypriot Baghdatis edged Lithuanian Ricardas Berankis 6-3, 7-6 (4).
The other semi-final features Spain’s Nicolas Almagro and second-seeded Tomas Berdych of the Czech Republic.
Almagro beat former No. 1 Lleyton Hewitt 6-1, 6-4, while Berdych defeated Russia’s Mikhail Youzhny 6-3, 7-6 (2) in the late match.
In a highly entertaining contest filled with baseline rallies, Berdych looked set for a comfortable win when he broke Youzhny early in the second set.
However, Youzhny raised his game and immediately broke back, after which the two players held serve until the tie-break. At that point, Berdych moved up a gear, taking a 5-0 lead before sealing his place in the semis.
Berdych is now 6-6 in head-to-head meetings with Youzhny.
Venus Willians beat top-seeded Roberta Vinci of Italy 7-6 (7/2), 6-4 on Friday to reach the semi-finals of the Luxembourg Open.
Playing in her first tournament since the US Open, Williams will next face Andrea Petkovic of Germany, who defeated Ksenia Pervak of Kazakhstan, 6-3, 6-2.