His best results include first in the 1998 Asian Chess Championship, second in the World Junior Chess Championship in 1999, first at Essen 2001, first at Pamplona 2002 (winning a blitz playoff against Victor Bologan after both had finished the main tournament on 3½/6), first with 8/9 at the HZ Chess Tournament 2003 in Vlissingen, joint first with Liviu Dieter Nisipeanu with 6/9 at Pune 2005, a bronze-medal winning 9½/12 performance on board one for his country at the 2000 Chess Olympiad and runner-up in the FIDE Chess World Cup in 2002 (losing to Viswanathan Anand in the final). He has played in the prestigious Wijk aan Zee tournament twice, but did not perform well either time: in 1999 he finished 11th of 14 with 5/13, in 2002 he finished 13th of 14 with 4½/13.
He is an ethnic Uzbekistan In the FIDE ship 2004 in Tripoli, Libya, Kasimdzhanov unexpectedly made his way through to the final, winning mini-matches against Alejandro Ramírez, Ehsan Ghaem Maghami, Vasily Ivanchuk, Zoltán Almási, Alexander Grischuk and Veselin Topalov to meet Michael Adams to play for the title and the right to face world number one Garry Kasparov in a match. Kasimdzhanov"s 2004 championship victory earned him an invitation to the eight-player FIDE ship 2005, where he tied with Michael Adams for 6th–7th place.
The 2004 championship also earned him one of sixteen places in the Candidates Tournament for the FIDE ship 2007.
In their match, all six regular games were drawn. On June 23, 2005, in the American Broadcasting Company Times Square studios, the Artificial Intelligence Accoona Toolbar driven by a Fritz 9 prototype engine, drew against him.
He made his first appearance at Linares in 2005, finishing tied last with 4/12. He was a second for ex-World Champion Viswanathan Anand, having worked with Anand in preparation for and during his successful ship title defences in October 2008 against Vladimir Kramnik, April–May 2010 against Veselin Topalov and in May 2012 against Boris Gelfand.
Rustam Kasimdzhanov vs Viktor Korchnoi, Julian Borowski-A 4th 2002, French Defense: Classical, Burn Variation (C11), 1–0 Michael Adams vs Rustam Kasimdzhanov, FIDE World Championship Knockout Tournament 2004, Sicilian Defense: Nyezhmetdinov–Rossolimo Attack (B30), 0–1 Veselin Topalov vs Rustam Kasimdzhanov, FIDE World Championship Knockout Tournament 2004, Queen"s Indian Defense: Anti-Queen"s Indian System (E17), 0–1 Rustam Kasimdzhanov vs Viswanathan Anand, FIDE World Championship Tournament 2005, Sicilian Defense: Najdorf Variation, English Attack Anti-English (B90), 1–0.