Alexander Zverev accelerated his ascension to number one in the world rankings for the first time in his career on Thursday as he beat Tallon Griekspoor 6-4, 7-5 to reach the quarter-finals of the Vienna Open.   The 29-year-old German was already assured of finishing the season top of the ATP rankings but, in the absence of current incumbent Jannik Sinner, two-time Grand Slam champion Zverev will now take that crown in next week’s updated standings after winning his opening two matches in the Austrian capital. Long seen as tennis’ nearly-man, Zverev has enjoyed a stellar 2026, breaking his major duck by…